I looked up charging times for air brakes on trains from a BNSF air brake and train handling rulebook. This chart is used when the locomotive doesn't have an airflow gauge. The longest train they had listed in the chart was 11,000 feet. A train of 230 53-foot long coal cars is 12,190 feet. Charging a train from atmospheric pressure to 90 pounds on a 11,000-foot train would take fro, 80 to 160 minutes.
Since the San Luis and Rio Grande wouldn't have a block of 230 cars coupled together the operation of coupling together, pumping air then shoving the cut into the next cut would take some time. When you throw in longer cars 3-mile long trains are possible.
Having trucks with air compressors ~ mobile yard air ~ to pre -charge the air might speed things up but people would still have to walk the train to make sure hoses are connected, angle cocks are open and hand brake set if they charged the brakes from a truck.
It would be something to watch if you had a lot of time.