Exactly. Some states have regulations as to how long a crossing can be blocked, but that's it. My state Arizona says
40-852. Allowing engine or car to remain upon public crossing; classification
An engineer, conductor or other employee or officer of a railroad company who permits a locomotive or cars to be or remain upon the crossing of a public highway over such railway so as to obstruct travel over the crossing for a period exceeding fifteen minutes, except in cases of unavoidable accident, is guilty of a class 2 misdemeanor.
There's no restrictions on time of day, and I'd be surprised if the railroads took that into account.