Ah, I see. In that case you may want to report this bug in the Pak64 sub-forum too. Depending on the response there I will think about introducing 1850-1900 vehicles independently of Pak64.
In general, however, I don't plan to add new features to Pak64.Exp which are not present in Pak64 standard -- so narrow gauge is unlikely to make an appearance. When I get around to making progress on my other project, Pak64.Unity, however, I would be very interested to discuss the addition of narrow gauge!