Pretty self explanatory. Right now to me trains sound no different when there outside to when they inside large train sheds. The horns have echo the trains themselves don’t to my knowledge. E.g: Manchester Piccadilly, London St Pancras, etc.
Pretty sure it is a thing in most routes in the game already. There's quite good ones at London pad and Birmingham new Street. Not sure of any other examples tho.
It’s fine in stations like Paddington but I can’t hear any echo coming from the train besides the horn in most cases. Especially Man Pic for example.
This probably falls within the same category as reverb in tunnels or clatter over bridges. Shuffled off to the too difficult department when MSTS did it 22 years ago and TSC also does it (though it can be horribly overdone on some routes).