You could follow the same track construction method that I have seen used for "O" Scale Traction. Code 125 is used for the rail and code 100 is placed on it's side next to the 125 to represent the girder. You can find information on this type on rail construction on the East Penn Traction Club web site. By up sizing the rail and following the same method of construction you should have what you are looking for.
I have played around with the geometries of some code 215 and code 250 rail from both Llagas and Sunset Valley and have not come up with a combination that gets the inner flange of the girder up even with the running railhead. I had used railhead to flange method back in my HO days. I even tried with some old O scale code 156 and HO code 100 I had laying around.
The best looking so far has been to lay a piece of code 100 on a spacer next to the code 250. The only drawback here is ther is no metal 'bottom' in the flangeway.
The East Penn site has been a good reference, I have been there many times.
Sometimes I struggle with that darned 10 foot rule.
The ABS is not a bad idea. It's cheaper than brass. When I built HO streetcar track, I just spiked the rail to a board and paved around it. Then, while the pavement was still wet, I pulled a flange tool through it. That left flangeways and it looks good.