"Tranny" is a common slur in enthusiast circles. It is a slur, although trans people have attempted to reclaim it in recent years. One of the reasons the slur is prevalent in the community is that there are a lot of trans women in particular in the space, and seeing trans women at events or spotting or bashing is more common than in most other walks of life. So, people have more "opportunity" to use it.
I have heard the word being used in a less directly hostile way by cis people but I think it is important to realise trans people consider it a slur. In the 90s the word "Paki" was often used as shorthand for south Asian people and my recollection is that it was a bit naughty but sure so what, your parents said it etc. There is much greater recognition that word is a slur now and "tranny" is on the same journey. It is an unpleasant word that normalises a hostile culture.
By way of anecdote, a lot of much older people use this word as inoffensive shorthand for (transistor) radio, and I have heard its use as a slur across age groups. Nonetheless the situation at
@Tetragon213 's heritage railway is unacceptable.