I know you aren't comfortable with the idea of weathering, but I think it would bring out the detail, more effectively than a satin varnish. I'm not sure what effect the varnish would have... the transfers, being smoother than the paint, might end up looking shinier... but you'd need to test it first on something expendable.
Martin Welch's book on weathering would be worth a look, but there are literally hundreds of video tutorials for the materials and techniques for military modelling, which apply equally to railway items.
These guys often demonstrate the techniques on a 'dead' model... would you consider making a quick mock-up of a wagon side, to practise on?