E.g. it would be unthinkable to build a railway tank car that was wider than the allowable bridge clearence, or had a wheel track that was not standard gauge. Changing standards in a mature system is a very serious decision. For Canadian Roads and Streets a 'P' or pasenger car establishes the minimum sized vehicle that should be able to be accommidated. The maximum sized vehicle is a tractor trailer. The current design vehicles can be viewed from the links below.
First approximations are usually linear using time or distance as the independent variable.
These 'rate' models are easily calibrated by measuring the total activity, cost, or revenue of a movement and then dividing by distance to obtain the average per kilometre, or time to obtain performance per hour, etc. Simple rate models can usually be improved by identifying a fixed component and removing it before dividing by the distance or time base. This produces a first order polynomial model. This type of model is often easily obtained from normal accounting information. If the activity is obviously non linear, then various transforms of the data may be used to establish an approximately linear relationship.