Hmm. The only time (as an adult) that I took the SMR to the top I had one of the diesels up, and then steam back down...
Distinctly the wrong way around to do it.
SMR isn’t really a heritage railway in the same way as most of the others. It was purposely built to perform the function it carries out today.
Having said that I’m not a fan of the modern carriages, but they have a high capacity which is presumably part of the business model. The steam carriages are rather nicer - thankfully they have taken the trouble to build pretty good replicas.
The SMR unfortunately gets a lot of bad reviews from visitors, often to do with weather, overcrowding and the price charged - as well as people who climb to the top then decide they can’t be bothered to walk down and expect to find a space waiting for them on the next train. I do think many people have unrealistic expectations - the railway runs at capacity during peak season and the logistics of taking people up and down a mountain are unique.
I went on it this year and was impressed. The best way to “do” it is to walk up and down, thus not having to worry about booking the railway in advance and taking a gamble on the weather, and allowing as much time as desired at the summit, then ride the railway up and down just to tick that off the list. The steam carriages are rather more comfortable and seats are reserved, but even then there seem to be squabbles about who sits where.
One could argue that the Ffestiniog is another railway which operates “in the now”, with many of its carriages being modern. However they do maintain a superb collection of vintage vehicles and there’s often one or two such vehicles on the top end of the train, mainly to provide extra capacity but with the benefit of keeping enthusiasts happy at the same time. Likewise the Welsh Highland.