Chevron, you'll get all the answers here. OO and HO are the same track gauge, but OO are 4mm to the foot scale. Don't ask me how these things ended up being a metric amount against an imperial one.... HO is smaller SCALE to accommodate American prototypes and you needn't worry about it unless you model USA / Canada. A "rake" of carriages, rather than a "rack", refers to a number of vehicles, e.g. a 6 coach rake, or a 4 car rake.
An 0.4.0 is a steam loco with zero leading wheels, four driving wheels and zero trailing wheels. A 2.6.4 has a pair of leading (small) wheels, six drivers, and four trailing ones. The RPSIs 461 is a 2.6.0 - look at it and you'll see.
Good luck with your modelling. Probably best to start with a basic set and get the feel of it, then you can develop it.