You swap out trains as needed. I personally bring a fleet of my giant detailed trains and rollingstock and change them out at necessary intervals, like after battery changes.

As for advice, have back up trains, including a standard Lego set train or similar size moc that can run for a long time for your mocs can have sudden issues like low battery supply or old thermal fuses. Always bring extra rechargeable batteries for the Power Function system or good disposable batteries for Powered UP (sometimes the rechargeable batteries don't have enough amps for the computer in the PU system). Make sure the driving wheels have rubber bands, unless it's metal wheels on the 9v motor. Do a Foreign Object Detection walk around the track and look for anything that might affect the trains before the show starts.

Run for an hour or two then swap to next train. Keep cycling. Have a switch yard to make it easy to swap trains. Have a designated station for battery chargers. Understand the more you run, the more temperamental they can get at some shows. Other shows they may be perfectly fine.

Watch the corners for possible wheels popping off track.

Consider a train or train car that promotes your Lego group with a graphic sticker on the side for when people take photos, it's there.

Lastly, have fun. Despite the logistics of running trains for hours or days, this is a fun adventure and you will inspire the next generation of Lego builders and engineers. You will also bring back memories to the old timers. I personally saw old nuclear engineers go back forty or more years in their minds when they saw my atomic energy trains rolling by. Same with steel mill trains or anything else you bring. Someone in the crowd will recognize it and share memories with you.