perhaps this is a stupid question but....
....well whatever. I was in my local layup and it is 3 tracks with about 30 cars total. mostly grain hoppers. i had been in the layup for like 3 hours when i heard a hiss come from under the line that i was painting. it was for only a few seconds and then nothing. i was wondering why it did this. the trains had been there for hours maybe even a day. there is no engines or yard for miles. can anyone explain this to me.