An open letter to a new starter
If you want to run trains of roughly LGB-size (or Playmobil size) in your garden, then you have a basic choice of two gauges. It's quite important to decide which gauge you want first, as this is probably the most costly thing to change.
LGB is "G Scale" - which is 45mm gauge. This roughly models metre-gauge, which is why it's popular in Europe and in the USA (where narrow gauge means 3 feet). Some off-the-shelf G-scale locos require powered rails, which then means you have to consider 240 volts and a track controller in the garden.
The other is "SM32" - which is 32mm gauge. This models 2-foot gauge, as used on the Ffestiniog, Talyllyn, the Lynton and Barnstaple etc. And therefore it's more prototypical for british modelling.
If you're more interested in "standard gauge" stock, you may want to try the Gauge-O guild, or Gauge 1, but I don't know much about them.
I started with a cheap LGB pack, and joined the association of 16mm narrow gauge modellers, and went along to a group meeting (in someone's garden). I found they all had SM32 lines, so I had to start all over again. There is a G-scale society too, but I don't know much about them. Re-gauging isn't all that difficult: you can easily get replacement wheels, and some locos can be regauged with just an allen key. (but not the LGB locos!)
There are a number of suppliers of kit, for both SM32 and G scale. This is a rather specialist hobby, so you'll probably get most stuff mail order. Most suppliers advertise in "Garden Rail" which you can get in WHSmith. Here are some websites from my bookmarks listGarden Railway Specialists, Princes Risborough
Mostly pro-G-scale, but the do sell SM32 as well.
Mainly pro-SM32, but they also do G-scale and Gauge 3 This website has a useful scale/gauge comparison diagram
Major manufacturer of live steam Garden Railway locos in G-scale and SM32
Association of 16mm Narrow gauge modellers
THE SM32 organisation.
I reckon you can develop a garden railway for about the same price as an indoor railway hobby. It has the added advantage of getting you outside, and sometimes the gardening "accidentally" gets done as you're playing trains - Bonus!