Found this classic number while searching for a different sketch......

"The Two Ronnies" (Ronnie Corbett and Ronnie Barker) were a classic comedy duo of 1970s British TV, and got some air time in syndication in other English-speaking countries (such as PBS in the USA). The two were noted for exquisite wordplay and magnificent lyrics in parodies, rivaling that of Gilbert and Sullivan. (Their most famous sketch, "Four Candles...." rivals "Who's on First?" as an all-time comedy classic.)

Here is a long (for them) musical number from their 1983 Christmas Special programme, featuring a "portmanteau" of the stereotype Welsh narrow-gauge railway systems, most notably shades of the Ffestiniog, Tallylyn, and Snowdon Mountain Railways:

https://www.youtube.com/watch?v=DcyQJbMO4_s

The model railway featured prominently throughout is the Dovey Valley Railway, a small OO9-scale (nominally 1/76th scale but running on N-scale track--see https://en.wikipedia.org/wiki/OO9 ) modular layout featuring a fantasy Welsh narrow-gauge railway. Said layout was famous in UK modelling circles for decades, and apparently still survives today, though this video depicts what was supposed to be its final public appearance on the "show circuit":

https://www.youtube.com/watch?v=YTF6nhWkU2M

2013 showing: https://www.youtube.com/watch?v=NKe8FKXyFr8

As for Corbett and Barker, they are somewhat exaggeratedly depicting the Welsh English dialect (Barker, the bigger one, was a seasoned expert on British linguistics and dialects), but the loco cab mock-up is brilliant--enough that people have speculated in the comments as to exactly what loco was the prototype.