If you like your toys to be accurate, you can buy scale models. Trains tend to come in the HO or OO scale gauges, both of these scales can run on the same track and it can be a little confusing. Hornby trains are HO gauge and you can check them out in this Hornby Train list
Another guage is the N gauge, which is a really teeny tiny scale. A bit to small to have fun with in my opinion.
But you can get other scale toys apart from trains, in fact you can get most vehicles as a scale mode these days. Especially in metal.