Hi rscott,
I assume that you are talking about the thru section adjacent to the passing siding between the switches.
I work it out using SCARM track software. It takes a combination of smaller pieces to fill the gaps.
For instance, there is no 4 or 6 inch combination in FasTrack. But , if you add five or ten inches, you can make 9 and 14 or 11 and 16.
Here is a very helpful chart some posted a while ago that has been very valuable in building FasTrack designs.
The other option, is to cut and make a custom sized piece and reduce the cost of many small fitters and track joints.
You should be fine using a combination.
If you tell me the total length between the two switches, I'll work it out for you.