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SOP 27

Standard Operating Procedure 

for 

Railroad Test Cars Using a Master Track Scale

1 Introduction

Purpose. 

This Standard Operation Procedure (SOP) describes the procedure to be followed for 

the calibration of railroad test cars on master track scales calibrated (certified) by the 

U.S. Department of Agriculture (USDA). By necessity, the SOP has been generalized 

to accommodate the variety of designs of master scales in the United States. The 

maximum railroad test car load is determined by the scale capacity and available length 

of track on each master track scale. The accuracy and precision of the calibration is 

determined by the reported values for each master track scale as part of the USDA 

railroad scale testing program, the proficiency and experience of the operator, and any 

additional control information maintained.

Detailed measurement ranges, standards, equipment, and uncertainties for this 

SOP are generally compiled in a separate document in the laboratory.

Limitations: The maximum railroad test car load shall not exceed the maximum 

calibration point listed on the master track scale calibration certificate or the 

nominal capacity of the master track scale.

Prerequisites

The master scale must have an up-to-date calibration along with a scale 

correction for the load being tested. The calibration must have 

demonstrated metrological traceability to the international system of 

units (SI), which may be to the SI through a National Metrology Institute 

such as NIST.

Access to the scale must be limited to authorized personnel only. If the 

master scale is owned by the railroad or other private entity, security 

must be maintained and controlled; registered keys to the facility must 

be provided.

Regularly scheduled calibration of all railway track scales is required

because the scale is used as a secondary standard. If a jurisdiction can 

supply certified test weights and appropriate delivery systems to 

conduct a substitution test, then the scale may be used as a comparison 

scale.
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The scale, process, and standards must be evaluated to ensure that the 

combined standard uncertainties for the intended level of calibration are 

sufficiently small. 

When mass standards are used with substitution methods, ensure the 

availability of suitable working standards or recently calibrated (and 

unused) field standards (e.g., NIST Class F, OIML Class M1) in 

quantities up to the capacity of the railroad test car that will be 

calibrated. All standards must have known conventional mass values 

with valid metrological traceability to the SI, which may be through 

laboratory working standards to the National Institute of Standards and 

Technology. In the case of substitution measurements, refer to NISTIR 

69691

, SOP 4, Double Substitution. 

The metrologist or specialist must be experienced in the operation of the 

master scale and have adequate means to deliver the test car to and from 

the scale. The metrologist is reminded of the importance of evaluating 

potential safety hazards prior to a calibration and taking adequate 

precautions to avoid personal injury or damage to the scale or the 

railroad test car. If the master scale is under the control of the railroad 

and/or located on railroad property, the work will need to be coordinated 

with railroad personnel.

Environmental conditions must be suitable for mass calibrations and 

within the recommended limits noted in Table 1. Equilibration times for 

railroad test cars are noted in Table 2. Deviation from these limits is 

permissible but may need to include additional corrections and 

uncertainty components due to possible convection currents. Railroad 

test cars that do not completely fit within the enclosed structure, 

allowing the doors to be closed, must be calibrated only when external 

environmental conditions are acceptable. The scale should be protected 

from the effects of wind and adverse weather. In some installations, 

drafts may still be prevalent, applying force to the scale. Every effort 

should be taken to eliminate the effects of air movement.

Table 1. Recommended environmental conditions.

Temperature Relative Humidity

18 °C to 27 °C 20 % to 80 %

Railroad test cars must be properly cleaned and painted and all repairs 

completed (as needed) prior to calibration. See section 2.3.3. for 

inspection requirements. 

 1 NISTIR 6969, Selected Laboratory and Measurement Practices, and Procedures to Support Basic Mass Calibrations.
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Railroad test cars must be completely dry, with no ice or moisture on 

the surface or under carriage of the car.

Experience has shown that a light coating of frost can cause errors in 

excess of the tolerance for the car. In heated facilities, the test car must 

be allowed sufficient time to warm to prevent condensation.

Table 2. Recommended stabilization times.

Temperature Range

22 oC (facility)

Temperature Range

72 oF (facility)

Minimum

Equilibration Time*

± 5 oC, 17 oC to 28 oC ± 10 oF, 62 oF to 82 oF 4 hours

< 17 oC and > 28 oC < 62 oF and > 82 oF 24 hours/overnight

* deviation is allowed if the known railroad test car temperature is at the facility temperature 

± 2.5 oC (5 oF).

Railroad test cars that repetitively demonstrate excessive “as found” 

out-of-tolerance conditions from the previous calibration date, may be 

required to be submitted on a more frequent basis; with every 

consideration being given to determine the cause of the variability.

Railroad test cars with unacceptable ballast; i.e., cement, pulverized 

lead, or other materials that absorb water or oxidize excessively, should 

be documented as to their instability. Repetitive tests demonstrating 

instability will warrant removal of the test car from service until the 

problem is corrected.

Stenciled weight of railroad test cars under USDA jurisdiction are 

required to be in 1,000 lb increments. Decals may be used for nonUSDA railroad test cars, which may also be set to the nearest 100 lb 

increments. 

2 Methodology

Summary

A railroad test car is calibrated by taking direct readings from the USDA calibrated 

railroad master track scale. The master track scale is calibrated by using calibrated 

10 000 lb mass standards and 10 000 lb weight cart that are maintained with 

traceability to the SI, and calibrated in a recognized2 facility in Clearing, Illinois.

 2 NIST Office of Weights and Measures, Handbook 143, Program Handbook provides recognition of compliance to program requirements 

associated with ISO/IEC 17025 and suitable evidence of metrological traceability. 
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Equipment / Apparatus

A track scale in good operating condition, with a current calibration 

certificate as noted in prerequisites. 

Barometer (if available), thermometer, and hygrometer with sufficiently 

small resolution, stability, and uncertainty (See NISTIR 69693, SOP 2) 

to record the monitored environmental conditions at the 1 % of air 

density level. Buoyancy corrections are not performed in this procedure;

however, environmental data is critical to demonstrate environmental 

stability during the calibration.

4

Correction or error weights adjusted to within NIST Class F5 tolerances 

with current calibration certificate(s).

Hand tools:

2.2.4.1 Wrenches;

2.2.4.2 Screw driver;

2.2.4.3 Wire cutters; and

2.2.4.4 Solvent to free rusty locks, as needed.

Means to safely move the railroad test car. 

Operational knowledge of railroad test cars including the braking 

system and operating controls is required. Only qualified personnel 

should operate and inspect the test car. Verification that the brakes are 

operative prior to movement of the vehicle is imperative.

Suitable adjustment material must be available for placement in the 

adjusting compartments.

Check list or inspection work sheet.

For reasons of safety and the observance of unstable operations, two 

people are recommended for performing the calibration. 

 3 NISTIR 6969, Selected Laboratory and Measurement Practices, and Procedures to Support Basic Mass Calibrations. 4

For most mass calibrations, the barometer, thermometer, and hygrometer are used to determine the air density at the time of the measurement. 

The air density is used to make an air buoyancy correction. The limits specified are recommended for high precision calibration. For this 

calibration, buoyancy corrections are not performed. 5 NIST Handbook 105-1, 1990, Specifications and Tolerances for Field Standard Weights (NIST Class F).
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Pre-calibration Inspections 

Inspect the Master Scale

2.3.1.1 Inspect the scale deck for wear and check for binds between the 

weigh rail and the approach rail.

2.3.1.2 Measure the gap between the weigh rail and the approach rail. 

If this distance is less than 1/8 in or more than 3/8 in based on 

requirements in the American Association of Railroads 

Handbook, a service technician must make adjustments prior to 

the calibration.

2.3.1.3 Inspect the scale pit for cleanliness and dryness.

2.3.1.4 Inspect the weigh beam, poise, butt connections, and 

counterpoise tip loop connections. While performing this 

inspection, put the blade edge of a screwdriver between the 

weigh beam and the side of the clevis at the pivot point, adjusting 

as necessary so there is equal distance on both sides between the 

clevis and the beam.

2.3.1.5 If using ratio weights, butt weights, or tip weights, confirm the 

ratio of the beam. This procedure can be useful to establish the 

as-found error. Railroad test cars that have been recently 

reconditioned may require several hundred pounds of 

adjustment material and correction to return them to nominal 

mass. Using ratio kits can reduce the amount of weight 

handling.

Inspect the Master Scale House

2.3.2.1 Note the following conditions and take action as warranted:

• Accumulated dirt or debris;

• Environmental conditions that may adversely affect performance;

• No load sensitivity: out of specification condition, look for 

physical bind, rodent or snake activity;

• “As found” balance condition; 

• Evidence of tampering or unauthorized entrance; and

• Unstable electronic indications. 

Inspect the Railroad Test Car

2.3.3.1 Determine the type of railroad test car (see Appendix A).

2.3.3.2 Note and record the stenciled mass value.
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2.3.3.3 Visually inspect the railroad test car.

Check for damage, obvious loose parts; or, have a qualified 

railroad employee inspect the railroad test car. Assess the 

cleanliness of the railroad test car and determine if steam 

cleaning or painting is required prior to calibration. Note any 

major leaks or accumulation of oil, grease, dirt, trash, or 

unnecessary cargo.

2.3.3.4 Check any available operation and maintenance records; note 

recent derailments, repairs or incidents that could affect the 

calibration of the railroad test car.

Note in particular:

• Brake shoes;

• Spring hangers;

• Grab irons;

• Padlocks in place;

• Air hoses in place;

• Hand brake working status;

• Pedestal bolts;

• Retaining blocks;

• Spring condition; and

• General condition.

2.3.3.5 For, self-propelled cars, inspect the following:

• Oil level in engine;

• Fluid level in hydraulic oil reservoir, assuring that the level 

is within operational range;

• Motor fuel levels, removing extra gas tanks not tagged or 

marked as being part of the car’s calibrated weight; and 

• All fluid systems for leaks.
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2.3.3.6 Check security seals, if present, for tampering or unauthorized 

entry to sealed compartments.

2.3.3.7 An inventory list should be securely posted in a conspicuous 

place to identify extraneous equipment that is to remain in the 

test car as part of the calibrated value. Items not on the list must 

be removed prior to calibration. Note: If there is a history of 

unauthorized repairs, permanently stamp or color code couplers, 

drawbars or other replaceable equipment, and note these 

conditions on the calibration certificate.

Calibration Procedure (Refer to NISTIR 6969, GMP 1, Reading Turning Points 

on an Equal Arm Balance if needed for beam scale readings.)

Zero balance the railroad track scale.

Set the scale with no load, no drop-weights or counterpoise weights, and 

the sliding poise set at 50.0 lb. If there is no poise, a 50 lb calibrated 

weight may be balanced on the deck of the scale. (If the test car has 

been recently reconditioned, a larger amount of test weight may be 

balanced in.) This is the reference indication, RI, for all measurements 

on the scale during the calibration. Arrest the beam, release and repeat 

the reading to show repeatability and evidence of correct scale operation 

prior to use in calibrating the railroad test car.

Apply the drop-weights or counterpoise weights to the beam in an 

amount appropriate for the size of the test car to be calibrated. (i.e., 

100 000 lb, 80 000 lb). Adjust the amount of drop or counterpoise 

weights until equilibrium is reached.

Position the test car on the center of the scale, taking note of the position 

markings on the rail if so provided. These markings are noted on the 

Master Scale calibration report and provide for corrected values at the 

various markings. Record scale indication as the railroad test car 

observation, SI.

Arrest the beam. Release and repeat reading. Record observations.

Adjust the test car by adding or removing stable, metal adjusting 

material (as needed).

Remove the test car, recheck the reference indication, noting any drift. 

Arrest, release and repeat. Record observations.

Reposition the test car on the center of the scale and move the sliding 

poise to attain equilibrium of the beam. This scale indication, SI,

determines the “as left” error. Arrest, release, and repeat. Record 

observations. 
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Remove the test car and recheck the reference indication and record 

observations, again, noting any drift. Arrest, release, and repeat. Record 

observation.

Marking 

Stencil the nominal value of the railroad test car and the date and 

location of calibration. Test cars under USDA jurisdiction are stenciled 

in 1,000 lb increments.

The metrologist may exercise discretion in marking or stamping 

removable or replacement components, such as drawbars or couplers.

3 Calculations (Measurement Equations)

Calculations. The calibrated scale is used as a secondary standard and the average of replicate 

mass values is used to assign the mass to the railroad test car. The scale indications, SI, and the 

reference point indications, RI provide the observed conventional mass difference. The scale 

corrections, Scorr, is subtracted from the observed difference to provide “as found” and/or “as 

left” conventional mass values. Where replicate measurements are made, the mean 

conventional mass is calculated and reported. 

Calculate the conventional mass, CMx. 

( ) CM S R S x I I corr =−− Eqn. (1)

Calculate the mean conventional mass, CCCCxx ������ .

1 2

2

x x

x

CM CM CM + = Eqn. (2)

NOTE: The Scorr needs to be subtracted whether the scale correction is positive or negative. 

For example, if the master scale reports lists a correction that is positive, this indicates that the 

scale is weighing heavy, and when negative it indicates that the scale is weighing light – as a 

negative scale reading correction. That means when you have a positive value for the scale 

correction it needs to be subtracted from the reading to determine the actual conventional mass 

value. 

4 Measurement Assurance

It is not practical to duplicate this calibration process with a check standard. Replicating the 

measurement process is encouraged. The procedure calls for repeated measurements, in which 

case a standard deviation chart may be used for measurement assurance. Measurement 

assurance of the scale operation is obtained through scale arrestments and verification of 

repeated scale reference readings for each weighing. If SOP 4 is used, replicate measurements 

will provide estimates of standard deviation that may be pooled if appropriate following 

techniques in NISTIR 6969, Section 8.
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5 Assignment of Uncertainty

The limits of expanded uncertainty, U, include estimates of the standard uncertainty of the 

mass standards used, us, estimates of the standard deviation of the measurement process, sp, 

and estimates of the effect of other components associated with this procedure. These estimates 

should be combined using the root-sum-squared method (RSS), and the expanded uncertainty, 

U, reported with a coverage factor to be determined based on the degrees of freedom, which if 

large enough will be 2, (k = 2), to give an approximate 95 percent level of confidence. See 

NISTIR 69696, SOP 29 for the complete standard operating procedure for calculating the 

uncertainty.

Uncertainty for the master track scale, uMS.

Calculate the average uncertainty for the master scale at the calibrated 

nominal mass to be used for the calibration of the railroad test car from the 

USDA calibration certificate. The USDA calibration certificates give errors 

for positions at each nominal value (and for sensitivity response) to be used 

during the calibration; however each point does not contain uncertainties as 

the average value is considered for applications.

Uncertainty for correction weights, tip or butt weights, or tare weights, us.

Obtain uncertainties for any correction or tare weights from their respective 

calibration certificates. 

Uncertainties for the measurement process and master track scale in use.

Standard deviation of the process 

Calculate the standard deviation of replicate measurements or use the values 

for repeatability from the sensitivity response and zero shift observations. 

Sensitivity Response (or Repeatability)

Obtain the sensitivity response value from the USDA calibration certificate 

for the master track scale. 

Zero Shift

Record observations during the calibration to determine the average shift from 

the established reference point.

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Table 3. Example Uncertainty Budget Table.

Uncertainty Component 

Description Symbol Source Typical Distribution

Uncertainty of the master scale 

calibration uMS USDA calibration certificate Normal divided by the coverage factor

Uncertainty of correction and 

tare weights us Calibration certificate Normal divided by the coverage factor

Standard deviation of the 

process

sp

Calculated from repeated 

measurements Normal

Sensitivity response sr

One half of the USDA 

calibration certificate reported 

value

Rectangular

Zero shift uz Observed data Rectangular

Calculate the combined uncertainty using standard root sum square methods 

and expand the uncertainty with a coverage factor that is associated with 

appropriate degrees of freedom. Values for repeatability of similar size railroad 

test cars may be pooled using techniques provided in NISTIR 6969, Section 8. 

Evaluation, Adjustment to Nominal, and Conformity Assessment

According to the Association of American Railroads Handbook, All test weight 

railroad cars are required to be maintained accurately to their designed or 

nominal weight values. For this purpose, each car weight shall be corrected as 

close to zero error and practical and certified within Class F tolerances as often 

as may be required. Railroad test cars should be adjusted to nominal 

conventional mass values (zero error) during calibration and must be 

maintained for use within 0.02 % of the nominal mass. The expanded (k = 2) 

combined uncertainty of the calibration must be less than one-third of the 

tolerance (0.0067 % of the nominal mass). 

Draft an Appropriate Uncertainty Statement

The uncertainty reported is the root sum square of the standard uncertainty of 

the standard, the standard deviation of the process, and the uncertainty 

associated with the buoyancy corrections, multiplied by a coverage factor of 2 

(k = 2) for an approximate 95 % confidence interval. Factors not considered in 

the evaluation: air buoyancy, magnetism, environmental variation. 

Note: Where inadequate degrees of freedom are available, k, is determined 

using the appropriate degrees of freedom and the 95.45 % column in the table 

from Appendix A of NISTIR 6969, SOP 29.

6 Certificate

Report results as described in SOP No. 1, Preparation of Calibration Certificates. Report the 

Conventional Mass, environmental conditions during the calibrations, and calculated 
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uncertainties. No correction for the effect of air buoyancy is to be assumed unless stated on the 

calibration certificate. In addition to requirements of SOP 1, the following information is 

included on the calibration certificate:

• Master Scale identity and its calibration date;

• Railroad test car identification;

• Stenciled conventional mass value;

• “As found” mass value (with explanation of adjustments, if appropriate); 

• “As left” mass value (railroad test cars that have demonstrated excessive wear from 

year to year may be left on the higher side of the tolerance to compensate for wear); 

and

• Signature of witnesses to the calibration, if appropriate and desired.
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Appendix A – Types of Railroad Test Cars

Monitor car. Essentially a modified freight car with approved lading to which 

field calibration has been performed. These cars cannot be verified on a railroad 

master scale. They are used only in emergency situations such as returning a 

scale to service pending the arrival of a certified test car.

Composite car. Each composite car is provided by design with a tool and/or 

cargo compartment, usually runs transversely through the car body which can 

be secured by lock or wire seal. Wheel base lengths (center of front axle to 

center of rear axle) shall not exceed 7 ft. Composite cars are calibrated on 

railroad track master scales. Standard composite car values range from 

30 000 lb to 110 000 lb in multiples of 10 000 lb.

Self-contained car. The self-contained type of test weight car has a metal body 

made up of either one or two castings mounted on a 7 ft wheel base with a space 

provided to contain a small number of 50 lb or smaller test weights. This 

compartment is called the adjustment cavity and is secured by lock or seal.

Compartment car. The compartment car is constructed of one or more casting 

or fabricated steel plates mounted on a 7 ft wheel base forming a shell that can 

be loaded with 50 lb or 100 lb weights to a value at least equal to the weight of 

the empty car. The compartment is secured by lock or seal.

Long wheel base car. The self-propelled hydraulic jack design is of average 

freight car length and employs four hydraulic jacks spaced 7 ft apart (to simulate 

a 7 ft wheel base for section testing) which are lowered to the scale section 

which is to be tested (repeated per number of sections). The self-propelled 

design allows the car to move autonomously onto the scale.

7 Acknowledgement

Material for this SOP was collected and reviewed between 1998 and 2018 with input from 

Bruce Adams, MN, Vic Gerber, WY, Marcus Harwitz, U.S. Department of Agriculture, R.C. 

Holmes, Union Pacific Railroad, F.J. Loyd, Jr., CSX Railroad, Gerald E. Davis, Conrail, Paul 

Hadyka, USDA-GIPSA, Bill Norrs, USDA-GIPSA, W.G. GeMeiner, Union Pacific Railroad, 

John Robinson, Association of American Railroads, Association of American Railroads 

Scale Handbook and GIPSA Weighing Handbook, Chapter 3, Railroad Track Scales.