I'm a bit disappointed at the generally negative attitude displayed towards Northern and a select few other operators providing some exposure for the traditional music of these shores, and taking the initiative to bring something different and potentially memorable to what were some of their more lightly loaded trains. There's a lot more to folk music these days than the almost unbearably cringeworthy finger-in-the-ear woollen jersey stuf that the genre has has been lumbered with the unfortunate image of.
The publicly popular oppression of the English tradition has gone on long enough! Power to the mandolin and the morris dancer!

However, on balance the TOCs undertaking such activities should ensure that these music trains are provided with a minimum of a two car unit, to give those not interested in the music somewhere to escape to as I imagine that it would be a bit of a daunting and unwelcome surprise for the unitiated not expecting a live music performance on their train.
Ideally, with patronage on many of these music trains and rural services in general at quite a high level nowadays, Northern would bolt on an additional 153 to the usual unit in order to act as the "music train" on services designated at such: If, for example, Northern did a monthly music train across four/five different lines at the rate of one a week (Hope Valley, Penistone, Whitby and the Tyne Valley, and possibly the S&C/Cumbrian Coast), then they would essentially have to source a single "wandering 153" (Though in practice it might not be the same unit every week) to act as the music train around the North.
However, if it ever happens, until the time comes that Northern have a bit of slack in their fleet after catering for all the existing capacity pressures, this isn't realistic when Northern need every unit they can get to increase capacity on the most heavily used services around their network.
But...if we're talking about
really ideal worlds here, then what I would love to see would be the music trains formed of three/four pressure ventilated mark 2s and hauled by a Type 2/Type 3 diesel sourced either from a local preserved line and giving special dispensation from NR to run over a particular stretch of NR metals (The NYMR already have a 25 cleared to run as far as Battersby Junction on the Whitby branch, all you'd need to do would be to clear it as far as Middlesborough...) or from one of the established spot-hire/charter operators (DRS, WCRC), operating an additional diagram so that a handful of return trips operate across the course of the day calling at the principal stations only, with a buffet acting as a real ale bar.

Ok, I know that that's not at all realistic, but I was only suggesting it in an
ideal world, and heck, if it provided an additional handful of services across the course of the day then I'd be reasonably happy with a bog standard 156 doing the same.