Two different operations two different opinions, what do you think is best. Boath operations run excursion trains primarily on the weekends but when Sunday comes around and the trains are going to be parked for the week there are two different opinions on what to do with the leftover air on the train. One operation dumps all the air from the tanks citing it helps reduce water in the tank, the other just leaves the air. What is the better option in your opinion?

My experience applies to cars and locomotives with on-board air compressors (trolley cars and electric locomotives), but we are instructed to always drain the air reservoirs after "pulling the pole" at the end of the day's operations. On humid days, they can accumulate a surprising amount of water. This probably isn't as big a concern with cars supplied through a brake pipe, but it probably doesn't hurt to drain the reservoirs (and then be sure to close the drain cock).

Which air are you talking about? Main or auxiliary reservoir?

We drain locomotives nightly, bottle cars with PM equipment, and draw down the trainline on everything else.

I checked with Dave Garcia, he recommends draining passenger car reservoirs. Also said it was a good idea for freight equipment, but may be difficult to do on some cars.

Bottling the air won't retain it overnight unless your system is VERY leak-tight.

The point of blowing down the air is to get rid of water (condensate) in the tanks. There is some question/doubt how effective this really is at stopping rust in the tank. However if you NEVER blew down, over months, the water level in the tank would begin to rise. Neglected long enough, you'd have a tank full enough of water that a splash could send a plug of water somewhere in your air system where water is not welcome. It could also cause freeze damage.

At trolley and railroad museums, I've seen three methods for dumping down tanks.

#1: Open the drain valve wide, and wonder why you get hit in the face with oily ballast.

#2: Arrange that the blast of air lands on an old pallet.

#3: Crack the valve slightly, so the venting is nonviolent, and walk away. Make reclosing those valves part of your startup procedure.

There is also a nifty device called a "thermionic drain valve". It is able to passively detect the presence of liquid water, and "blow" only when it is present. You hear these on modern locomotives making a very staccato and intermittent "FT" sound, much shorter than the "PSSSHT" of a blowdown which cycles with the compressor. The units are very modestly priced, well under $200 I would expect.

You need to get the condensate where it shows up. It will mostly be in the first tank past the compressor/intercooler. Some in the second tank. By the time it goes through the brake valve, it should be pretty dry. In any case you can't blow down freight car tanks, because the dump valve has to be at the bottom, and freight cars have no such valve. Clearly, they don't fill with water...