Is the line close enough that you can hear the trains? If so, that can be your guide. Just make a note of when you hear them for a few days, a couple of weeks is better, as there may be regular variations from day to day.
You did not give any indication of the normal frequency of trains on the line, or where you are. Some hints fo that nature might help.
If the line is really busy, then any time could be as good as any other. If not, that is another story. then you need to determine if there is a pattern.
In the past many railroad companies published freight schedules, but apparently not anymore. Even when they did, for the most part there was a large dose of approximation for all but a few "flagship" type freights.
If you have a scanner, you may be able to hear them talking if you know their frequency or range of possibilities, and if there happens to be a need for the crews to talk when they are in your area. I really know nothing about this subject.
