Low Air Pressure Before the Downgrade and the Crossing
At 6:55 a.m., during your pre-trip on an air-brake equipped 40-foot bus, the engine has been running about four minutes. The primary gauge reads 72 psi and the low-air warning buzzer has just sounded; pressure was 95 psi two minutes ago. You hear no obvious leak, but the compressor seems to be cycling frequently. Thirty students will be waiting at the first stop in 15 minutes, and your route includes a 3/4-mile 6 percent downgrade and a passive railroad crossing with a stop sign and 700-foot sight distance. The spare bus is in the lot but the mechanic is not on duty for another 30 minutes. Your dispatcher is reachable by radio. The bus has a current annual inspection sticker.
1. Do you take the bus, wait 30 minutes, or do something else? Use the gauge numbers in your reasoning.
Read the worked answer
Do not take the bus. Normal air-brake pressure is roughly 100–125 psi; the low-air warning must come on before pressure falls to 60 psi, and the spring brakes apply somewhere around 20–45 psi. You started at 95 and lost 23 psi in about two minutes with the engine running, and the buzzer is already on. That is a leak or a failing compressor, and the bus cannot store enough air for a 3/4-mile 6 percent downgrade where you will be on the brakes continuously. It also cannot guarantee a full stop at the railroad crossing. Waiting 30 minutes for the mechanic beats moving the bus, but the students still need service, so radio dispatch now: request the spare or a mechanic, report the gauge readings and that you are placing the bus out of service, and ask the school to hold the first stop. Do not top off the air and try the route. Document the readings, times, and buzzer.
2. Suppose the warning comes on mid-route 300 feet before that passive crossing and you hear a train horn. What do you do?
Read the worked answer
Do not enter the crossing. A passive crossing has only a stop sign, and 700 feet of sight distance is not enough if your brakes are losing pressure. School buses stop 15 to 50 feet from the nearest rail, but if the bus will not hold pressure, stop even earlier, on the approach, where the bus is not on the tracks and students can be moved away from the rails. Set the parking brake, put the bus in park, keep the radio, and get the students off and at least 100 feet from the track, upwind and away from the road; then call 911 and dispatch. Never stop on the tracks and never try to beat a train-a train at 50 mph needs more than a mile to stop. If the bus does stall on the tracks, evacuate immediately and move away at an angle, not parallel to the rails.
3. The next morning the same crossing's gate is down with no train visible. What is the correct action, and why is going around wrong?
Read the worked answer
Treat a lowered gate as a stop signal no matter how empty the track looks. Stop, open the door, look and listen, but do not drive around or under the gate. A gate can stick, a signal can fail, and a train can be approaching from a direction you cannot see yet, especially at a crossing with limited sight distance. Radio dispatch, report the malfunction to the railroad using the blue emergency sign number if one is posted, and call 911 if the crossing is malfunctioning and traffic is at risk. Wait for the signal to clear, or for a police officer or railroad employee to direct you across in person. Do not accept a radio instruction to go around the gate. Document the delay and the time you reported it so the school knows why the route was late.