There is this. While not listed in the catalog, not sure why, it does do what you want.
From the Fall 2016 Legacy Users Group Meeting...
We showed some “un-cataloged” items for Legacy PDI network (for some reason I keep calling it PID). The first being a PDI network “coupler”. This small passive device will allow 2 PDI cables to be joined without needing an LCS device. This will come in handy to extend a PDI cable or when a LCS device is removed the cable can be joined up. This is available through Lionel parts. The PN is 691JOINA00.