Date: 201901
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Takeoff / Launch
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Second aircraft: Regional Jet CL65; Undifferentiated or Other Model
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Communication Breakdown
Second reporter function: First Officer; Pilot Not Flying
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Less Severe
Detector: Person Flight Crew
When detected: Taxi
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure; Staffing
Primary problem: Staffing

Narrative: Standard taxi to XXR. Flight in an unusually long departure line up of approximately eight to ten aircraft ahead and the same behind our flight. Tower Controller stated over tower frequency that no line up and wait clearances would be given because he was working both ground and tower frequencies. Eventually Aircraft Y was given takeoff clearance ahead of us. Then we were given a takeoff clearance from XXR. I taxied onto the runway slowly to give time for Aircraft Y to rotate off runway. As I lined up for takeoff both the FO (First Officer) and myself saw an aircraft crossing our runway. I brought the aircraft to a stop not having yet set takeoff power. It was then the RWLS lights (Runway Status Lights) came on (I thought a little late). My FO told the tower we would hold and wait. The tower hesitated and said something about line up and wait. The crossing aircraft cleared our runway and we were reissued another take off clearance. Rest of flight normal.I believe this event occurred because the ATC Controller was working the west side of the airport's two frequencies by himself during an unusually busy bank time late at night.I am not sure why the west side of ZZZ was so busy at that time. It did not seem normal. Even a great ATC controller cannot work this much traffic alone. Keep additional controllers around if there are unusual traffic events occurring.


Second narrative: Our flight was operating normally until we reached the takeoff pad for XXR. We were approximately 8 to 10 in line for takeoff. I had been flying this flight most of the month before and had not seen this much traffic in my previous experiences. I do remember the tower controller saying that he couldn't do line up and waits because he was operating both tower and ground control. I noticed even as we were number one for takeoff that there were still 8 or more behind us waiting for takeoff.Eventually we were number one for takeoff. Aircraft Y took off in front of us. The clearance for takeoff was issued prior to the previous flights rotation. The CA (Captain) was moving slow as to allow the previous flight to rotate and clear the runway. As we were lining up; I saw an aircraft crossing the runway. I said to the CA to stop and wait. Just as I said this the RWLS (Runway Status Lights) system lit up. I transmitted that we would hold and wait. Tower hesitated and said something about line up and wait. The traffic cleared the runway. Tower reissued the takeoff clearance. We responded and performed a normal takeoff. The fight proceed normally thru the rest of the flight.The event was the result of an undermanned tower. [Company] continues to expand the number of late night and extremely early departures. If there are any delays the ATC system and other services lack the ability to handle the work. Thankfully the RWLS is in place in the event we did not see the crossing traffic.


Synopsis: A321 flight crew reported being cleared for takeoff with another aircraft crossing the runway.