Stourbridge to Walsall via Dudley should be reopened to rail and not be used for the Midland Metro. The potential for taking cars off the road is there already! At present the only way by train from Stourbridge to Wolverhampton is via Brum!! (or 80 mins by bus!)
A large car park for park and ride could be built at Dudley.The Merry Hill shopping centre would also benefit. This is a highly populated area where the only public transport option is bus!
Or how about Bangor to Afon Wen via Caernarfon? Imagine how the Cambrian Coast would benefit - especially when the Welsh Highland Railway is complete. You could complete the circle from Caernarfon to Porthmadog by narrow gauge then standard back via Criccieth.
Its a pity the tunnel in Caernarfon is now a road, a supermarket is also in the way and the WHR has stolen the formation south of the town!! Oh well, it was an idea!!