This detail part is intended for use with the recent "New-Tool" Bachmann 4-4-0s.
This is a condensed boiler backhead, which preserves the electrical plug below the cab floor, but allows for additional in-cab detailing. Modelers will have to remove the existing plate weights that come installed factory from their model, so it is highly recommended alternative weight methods are pursued so that you can retain traction over your rear driver.
The detail should slip over the existing plain-surface backhead, with a tension fit, no gluing required!
