It's probably more to do with the difficulty in creating a good sound project. The noise these fantastic machines created is very much non standard, just go to youtube and have a listen to one or two. The problem is created because you can here a distinctive rapid chuff and the noise from the chain drives, which will not be easy to replicate simply by recording the real thing as in reality the noise is not synchronised. e.g. when coasting very light chuffs and a noisy chain but when driven the chain drive is in the background only.
If this can be achieved authentically it will re-create the illusion of the full size machine.
No one yet seems to offer a good solution on Zimo or ESU - so I think it must be difficult.
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