How about a "drawbridge" over the main track? I've seen photos of such on narrow gauge sugar lines in Queensland and on logging lines in Ukraine. An alternate solution would be to bring the secondary rails up close to the main rails at a slightly higher elevation so your flanges bump just a bit on the main track. If you laid some stout planks in the gauge of the main track your wheels would ride on their flanges over to the rail on the opposite side. Just keep your speed below a hundred and you should be ok!