The Battle of the Gauges
Trains run on tracks. Tracks are made of lots of bits (if you want more info you will need to visit a serious site). Railway people don't call them tracks, they name them LINES. Hence railway line not railway track - unless of course it's a single line then it's called a single track (or visa versa).
The tracks are fitted with fixed rails a uniform distance apart - although, weirdly, they do meet at the horizon. The distance between the inside edges is called the gauge. For all sorts of reasons, but chiefly because engineers can never agree with one another, the gauge differs from railway to railway.
Railway folks use slang terms to describe the gauge the picture explains this. So, hopefully that's cleared that up.
Of course, a four-foot track is wider than four foot; and, by the same token, a two-foot track is less than two feet. But then you probably guessed that. We haven't even mentioned Mr Brunel's seven foot and one-quarter inch Broad Gauge! That would involve some serious fawning so better move on hey?
The wheels on the train go round and round; round and round; round and - oh, we've stopped at a red signal.
Here's a helpful guide.