Trying to understand train schedules between Belgrade and Bar, Montenegro - are there any websites in English ?
Sure, passengers can use either
Serbian Railways website timetable page or
Montenegro Railways schedule. In case you plan to travel during the summer, then also additional trains will be available
Ideally interested in going from Belgrade to Bar and then to Sarajevo and then back to Belgrade - possible by train ?
Well, traveling by train from Montenegro to Sarajevo isn't really an option, since there is no rail line, however there are plenty of direct buses (daytime and overnight). It seems that during summer (high-season) there is even a direct one from Ulcinj-Bar-Sarajevo, but nowadays (low-season) there are buses only from Podgorica and western part of Montenegro (Herceg-Novi).
Taking a train between Sarajevo and Belgrade is also quite problematic, direct one was canceled some time ago and probably won't be reintroduced any time soon, while traveling with change via Zagreb isn't really a great option. So it's either multiple changes of bus+train or direct bus Sarajevo-Belgrade