Richard J. wrote:

As you say, the apostrophe has a circular ball at the top, but the dot
under the T of "ST." is square. Also, several letters are narrower than
Johnston, and the E and K have subtly different proportions. Looks like an
amateur attempt to copy Johnston. I wonder why.

Because the real thing was hard to get hold of?

I've heard of one such font called SubwayLondon. Could it be that?