Date: 202105
Local time of day: 1201-1800
Location: DCA.Airport, DC
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class B DCA
ATC / advisory: Tower DCA
Second aircraft: Commercial Fixed Wing
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Situational Awareness
Second reporter function: Captain; Pilot Flying
Anomaly: Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; No Specific Anomaly Occurred Unwanted Situation
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors; Environment - Non Weather Related
Primary problem: Human Factors

Narrative: After being cleared for takeoff on Runway 1 in DCA; the Captain transferred the controls to me. Shortly after the '80' knot callout and my response; I noticed Aircraft Y approaching an intersection ahead and it appeared they were moving faster than if they planned to hold short of the runway we were departing. I focused on flying the aircraft while closely monitoring Aircraft Y. Just as I was considering taking action; the aircraft came to a quick stop short of the runway. Around the exact same time the Tower Controller transmitted 'Aircraft X!' with no further instructions. We continued the takeoff uneventfully and Tower gave us their phone number to call upon landing in ZZZ; assuring us we were not in trouble or at fault but wanted to hear our side of the situation. The Captain asked me if I saw the aircraft and after a brief discussion it was obvious we both saw the same situation unfold.I am unaware of Aircraft Y's communication with ATC. With that said; I'm assuming it was either a lack of situational awareness on their part or ATC giving incorrect instructions.I do believe the system in place will normally prevent these occurrences. Maintaining situational awareness. Verifying clearance prior to crossing an active runway. Both pilots being heads-up. Physically checking the runway for aircraft should be the last barrier.


Second narrative: We were cleared for takeoff on Runway 01. I turned control of the aircraft over to the First Officer (FO); and after he applied takeoff power I took control of the throttles. Several seconds later I made the '80' call; which the FO acknowledged. About this time I noticed an aircraft (Aircraft Y; I think) up ahead approaching Runway 01 perpendicularly on a taxiway (probably Taxiway G; but maybe M) at a fairly rapid taxi speed. I focused on the aircraft and began to wonder if it was going to stop as it got closer to the runway environment with no sign of slowing. At this point I mentally prepared for performing a rejected takeoff. Just as the aircraft reached the point where I was thinking I would reject the takeoff I noticed the aircraft stop with a pronounced 'dipping of the nose' from hard braking. At this point I knew the aircraft had stopped and takeoff could continue. I estimate our speed to be about 120-130 knots at this point. A second or two later the Tower Controller made a somewhat excited call; 'Aircraft X!...no further transmission'. As we passed the other aircraft I looked over and couldn't determine if it was over the hold short lines; but could tell it was very close. We continued the takeoff and shortly after getting airborne; the Tower Controller asked us if we had the number to call the tower when reaching destination. After retracting gear and flaps I asked the FO 'Did you see that other aircraft?' He replied he did and after further discussion I determined he saw basically the same things I did (aircraft approaching the runway; considering a rejected takeoff; 'dipping of the nose' with hard braking; knowing the aircraft had indeed stopped and comfortable continuing the takeoff). I called the tower when reaching ZZZ. The Controller taking my call informed me the other aircraft had entered the runway environment and asked if we noticed anything. I replied we did; but continued the takeoff because we knew the aircraft had completely stopped and we couldn't determine if it was over line or just very close. The person taking the call also mentioned the Tower Controller started to issue a rejected takeoff command to us but didn't finish because the Ground Controller confirmed the encroaching aircraft had stopped; hence the 'Aircraft X...'SMGCS lighting installed at DCA and used in all weather conditions; not just low visibility. Red lights are very effective in reminding pilots they are not cleared to cross/enter a runway.


Synopsis: Air carrier flight crew reported seeing an aircraft taxiing too fast towards them and were readying to take evasive action before the other aircraft finally stopped.