The THSR was quite an achievement - extensive tunnelling was involved and earthquake protection needed. I've been travelling to Taiwan since 1995 and lived there for a while. It really did revolutionise travel. For the first time people living in, say, Hsinchu or Taichung could pop to Taipei to meet friends, do some shopping and be home in time for tea. 2 hour coach journeys became 30 minute train journeys. Unlike the Japanese system (on which it's based), fares are very affordable. Also simple - if only UK intercity fares could be like that.
The MRT has improved things for Taipei dwellers, but bear in mind that there are different ways of romanising Chinese. You can sometimes find that the spelling on the platform signage differs from that on a map or that on a map inside the train. The reason is political - different political parties favouring different methods, though recently things have improved and there is more consistency. I was on an MRT during an earthquake. The driver proceeded to the next station and told people that he would open the doors but that we should remain on the train. (Taiwanese do as they are told.) It was bizarre - we could feel nothing, but the station was moving up and down, then side-to-side before settling down.
The Alishan mountain railway is worth a look. It is possible steam will be removed for good soon (if it hasn't already).
The worst thing about Taiwan from a rail enthusiast's point of view is that there is no railway connecting the main airport (Taoyuan) to Taipei. Taoyuan airport is otherwise friendly, modern and efficient. I am told that the bus companies (which have political connections) feared the loss of business. The HSR line passes nearby, but to access it you have to get a shuttle bus (cheap, not free) to Taoyuan station. They really should have built a spur. Daft thing is that Taoyuan station isn't really in Taoyuan either. Unfortunately that's true of a number of THSR stations (eg Taichung) - you have to use a taxi or irregular shuttle bus to get into the centre.
An MRT extension is being built which will reach the airport soon.
Thoroughly recommend EVA Air for getting to Taiwan if LHR is good for you. Direct flight (stops in Bangkok). I use them 2 or 3 times a year without a hitch. "Elite" class gives you extra comfort if you can get it at a reasonable price (2-4-2 layout, 38 to 40" pitch). Their business class ("Royal Laurel") is fully lie flat and of course expensive unless you can upgrade with star alliance miles or something.
English is not very widely spoken. Taiwan is off the tourist trail except for mainland Chinese and Japanese visiting their old colony. (Perhaps their only ex-colony where their involvement is not universally hated.) But Taiwan is generally scam-free and civilised. If you get stuck, usually someone will be around who can speak English and will try to help.
If you're into IT products, they have some lovely malls where you can pick up cheap (and genuine) gear.