Nice loco Rune! Congratulations!
In answer to your question about 2.4GHz versus 40MHz, there are three main advantages as far as I'm concerned.
You don't need to worry about anybody else also using 2.4GHz. The systems don't interfere because, in effect, they automatically choose different frequencies, whereas you need to make sure you have different frequency crystals when using 40MHz.
The aerial on the transmitter won't poke anyone's eye out because it's only a few inches long (15cm or so).
And perhaps most importantly, 2.4GHz isn't prone to "glitching" like 40MHz. It seems to be more of a problem with tender locos but it can be a nuisance with any loco.
Best wishes,
Steve