"if you book in advance you have a seat reserved."

Except that each train has a sign saying reservations are not guaranteed. I commute on UK inter-city trains every week day and reservations are frequently cancelled because of overcrowding, carriage failures, 2 trains being combined into 1 etc.

My advice would be to avoid commuter times; you may not be able to board an overcrowded train at all, even with a reservation, and you will be very unpopular if you have luggage/guitar cases etc at peak times.
If you have to travel earlier to avoid rush hour problems, this could result in a lot of time hanging around before your gig.