I don't work for any of the TOC's you mention but I will try to explain the jobs although Damons link is very useful for anyone who wants advice about getting into the rail industry.
Platform staff generally refers the old RO2 grade. This put broadly means dispatching trains, keeping the station tidy and providing help for passengers. Its not a bad position, the only problem is you are outdoors for a lot of the time which isn't very nice in the winter and it can be quite hard being on your feet for 6-7 hours. Its can also be testing when the train service goes up the shoot as you will be in the firing line. For many TOC's this is an entry position and is a good way of getting your foot in the door.
Conductor means you are responsible for operating the doors and then going through the train checking and selling tickets. I am pretty sure that unlike Guards you don't get any technical knowledge of the trains you are working. The checking/selling tickets can be pretty difficult if your TOC has any rules about buying tickets before travelling as many expect you to sell full fare tickets only and as TPE is a First company then I would think that it would have. Its a interesting job and is a good position to take if you wanted to go on to train driver as you would have PTS to work as a conductor and experience of working on trains.
Both positions are classed as safety critical which means you would have to undertake a safety critical medical. For conductor you would also take a enhanced assessment which will include the "dot test" which tests your concentration. There is a link on Railchat to a website which has software for you to practice. It you think you won't get through a safety critical medical then ticket office and clerical work would be the best line to go down plus most Revenue Protection grades these days are not classified as safety critical even though you would be working on trains.
Hope that helps!
