I think that people on the correct train with a reservation should HAVE to sit in their seat, this WILL happen when I am Queen it's really aggravating to have all the non reserved seats filled and a train of empty "reserved" ones.
AND to facilitate my Queen Laws, the train company will have to pull up their organisational socks and properly identify where you have to stand on the platform for the required carriage, some stations do this with letters so you know, but then they run the bloody train backwards. How hard can it be to add a little something to the announcements??
"bing bong....the next train at platform 1 is the train to Timbuctoo, it is a four coach train, coach A is located at the front/back...bing bong!"
Also, it'd hardly kill the train guard to remove them as they are unoccupied, so as you leave the station and tickets are checked, reservations are also checked and removed if unoccupied. SIMPLES. Seats for everyone!
I also do not really see why, in this day and age of high tech shenanigans, you cannot book yourself in, as you do for air travel, esp for all the tickets with travel options, that will let you travel on many trains but only reserve on the one train which is a nonsense