I had a quick Google, and found this:Whilst I fully understand the tourism requirements of running steam excursions especially on the coastal Cambrian I also understand the "problem" that ERTMS has brought. My understanding was that somewhere in Europe a portable ERTMS unit was in use.
http://www.rail.co.uk/rail-news/2011/ertms/
Some of the railway press suggested that a Swedish portable ERTMS system could be used on steam locomotives, but this system is not considered by the rail industry to be a genuine solution, and rumours to the contrary should be ignored Network Rail said.
Not DMUs, or steam, but I think I read somewhere that West Coast Railways weren't happy with the availablity of the four Network Rail yellow 37s a.k.a. 97s to haul charters on the Cambrian and are planning to fit some of their own 37s with ERTMS.I do wonder whether one of the "specialist" companies couldn't invest in a couple of Heritage DMU's, modify them for ERTMS, and use those on the Cambrian. I'm sure they would be popular, especially if the driver's blinds were kept up.