hi, i think these information will help you very much:
well , you have first to ask for BIR BOURAGBA TRAIN STATION:
timetable: here are 3 times for departure: 6:52 morning,11:00 morning and13:56.
hammamet- eljem: is about 2 hours.
but if you want to return from eljem to hammamet: 12:00 mid-day,14:10 and 19:15. (tunisian time+1 GMT). going to eljem is about 7d000 (less than 3 pounds) .but taxi is more expensive 100d(almost 40p) of course per day,i think that going by train is better for you and then you can come back to hammamet by taking your train at 19:15 night.
I think i gave you everything you need.
good luck
