Hi Jetlagged
My first question would be are TfL still paying you? If yes, have you signed a contract agreement? If yes, then it will be very hard for the company to release you unless you have been at the heart of a major accident/incident or gross misconduct, and also for drug and alcohol use whilst on duty or after random screening. Remember that once they've agreed to take you on board with a contract signed, they've decided per the signed agreement to invest in your career and development until you meet TfL standards, conduct, rules and regulations as a train operator or whatever to want to become later on with the company.
That's the reason why people are often been advised not to give a formal notice period to your current employer to leave and join another unless you have signed a contract or agreement with the latter.
I hope this helps. I wish you all the best.