Again my personal favourite is SimSig. This collection of programs has recently become free to download and many a happy hour can be spent signalling trains around areas as complex as Waterloo and Clapham Junction, Didcot, Swindon and Westbury to name but a few.
Some have a choice of timetables and the level of complexity is also user set, so you can start off with no problems and delays and as your experience increases so can the level of difficulty.
The following are a few screenshots that I've taken of the Swindon simulation to illustrate the appearance, but visit the SimSig site for much more detailed information.
Also attached to the SimSig site is a forum dedicated to the topic, and there's always a great deal of information being exchanged and news of new developments. It's also very 'user friendly' and users new to the simulation are always welcomed warmly and information and help to them is freely given.