Is it possible he's a volunteer on a preserved railway and has one legitimately as such?
On the other hand a T key is just a square chunk of metal, sometimes with a hole in it. The railway really should be much more sophisticated than that these days, it's not as if it's hard to render them all pretty much useless by using actually secure methods of access, e.g. proximity cards on electronic locks or even normal house or car type keys! It's not hard, and such locks work for ages on batteries, as per pretty much every hotel door in the world these days!
(There are also some house keys that will open non-pin-fitted T-key locks. I "might" have used mine to open windows on a Class 166 when the temperature inside got well over 35 degrees due to the "aircon" not working, and I would happily stand up in Court to defend having done that had any member of staff got shirty rather than apologetic about the situation).