I was tempted to base my projected layout on a standard gauge branch from Welshpool to Llanfair Caereinon, and went into full research mode as a change from building things. It's attraction was the dual involvement of the LNWR and GWR before the Cambrian was formed, and the direct link along the 'Cambrian mainline' with Shrewsbury.
Photos exist of L&YR A Class 0-6-0's and Fowler 2-6-4T's on passenger trains, and Midland 3F Tender engines on local goods....all in BR days. These engines bypassed Oswestry at Buttington Junction.. A cab journey along the current narrow gauge W&L showed the nature of the line, its 1-in-29 inclines and the lack of visible settlements along the route.
But on reflection, it is best to stick with what I started. A MR 3F on the goods is tempting...