Miniature railway engines

[Spaceman224456]

So, I've been thinking about what scale the miniature railway engines would be and I would like to see other people's input on what a potential miniature railway engine's scale would be?

[TTL]

I mean to match the rest of the main range? They still be OO scaled, but they would be running on Z gauge track, closest thing to 15 inch track in OO scale.