I’ve come across a couple of sites that recommend cutting a section out of a stump and setting the rail vertically. You’ll be working on the cross-section of the thick part of the rail. They say this is about as large as the “sweet spot” on most anvils and, with the bulk of the rails’ mass under the hammer, this feels like working on an anvil two to three times as heavy. I’ll be giving this a try.
Llewellyn o’r Glynn