Do you ever think about railroad sidings?
Not the big rail yards, but the ones we see along highways while driving. There will be a main line with occasional sidings next to it. Sometimes there will be trains pulled over on a siding, waiting for another train to pass so they can proceed on a clear track. Sometimes there is just a line of rail cars parked there.
I notice that the two tracks always seem to be on different grades; one is always lower than the other. My guess is that it’s a safety issue and the lower track is always the siding. If cars are left unattended on a siding and the brakes fail, don’t want to take a chance on them rolling downhill to the main line and causing a crash. They’re less likely to roll “up” to a main line track if the siding is lower. I googled it to see if I was right. I like to arrive at my answer before I start the fact-finding. Google says the sidings are lower because they don’t have to hold as much weight as the main line, so they put less rock ballast under the siding track and using less ballast saves money.