TfL is short for Transport for London, the government body that runs public transport in London — the Underground (the Tube), the buses, the Overground, the Docklands Light Railway, the trams, and the Elizabeth line. It also looks after the main roads, the traffic lights, the cycle hire bikes and the congestion charge, so it is bigger than just trains. In everyday use you will hear it in sentences like 'TfL announced a strike' or 'check the TfL website'. Notice the spelling: capital T, small f, capital L. That small f is deliberate, because it stands for the little word 'for', and TfL is fussy about it in its own writing, though newspapers sometimes print TFL anyway. Grammatically it is usually treated as one thing — 'TfL is raising fares' — but British English sometimes treats organisations as plural, so 'TfL are raising fares' also turns up in speech. The word is really only useful in Britain; outside London, few people would know it.