I've got a 12VDC input thats being used to create 5V to run a AVR and 8V to run 8DC motors. I need 10VAC as well to run some accessories. Basically they are just switches controlled with a solenoid. So the draw on the 10VAC will be minimal.
It seems to me you could produce an approximation of 10V AC from your 12V input if you can drive an H-bridge with the right timings.
see "modified sine wave"
It can be done. There are two divergent paths -- the first: If you have plenty of excess 12V capacity (more than a few amperes worth), you could simply buy a small inverter at an auto jobber, and power a garden variety Märklin compatible accessory transformer. This will be horribly inefficient, but requires no designing or construction.
The second, much more complex path, would be to design a custom inverter.
Something that would take 12V and regulate it down to 10V, and feed that into a transistor array called an "H-Bridge" to generate 10Vrms (more or less) AC.