The medical is nothing to stress about as long as you're reasonably healthy. Just a questionnaire on medical history, blood pressure, BMI, breathalyser/urine sample for drugs, eyesight, colour blindness, and hearing test. The hearing test is just pressing a button every time you hear a beep.
You'll know on the day about everything except for your drug screen, but you should know the answer anyway.
The job role
That's basically my job. I'm in a Faulting and Maintenance team for signalling, which means routine mtce until a fault comes in, then fault finding and rectifying it.
Some areas have dedicated Faulting teams, but generally expect to be doing some mtce too. If you're OK with shift work then it sounds like what you're looking for.
Put in for Signalling, with E&P (Electrification & Plant) as a back up as that's electrical as well, but higher voltage, and I think a bit less on track.