I found a BicyclePA Route J2 today and followed it for much of its length rather than taken the usual way between Harrisburg and Baltimore. It begins in Lemoyne at Market St. & 3rd St. and criss-crosses southwesterly to PA 94 & Old US 15 in York Springs. It continues southward along Old 15, but I stopped following it at that point.
PennDOT's site shows only the single-letter routes, like J. Are there many of these child routes? Perhaps at least a J1 somewhere else along J?
I've seen a few here and there, though I can't remember offhand where...
There's a bicycle route S near "Old Pa 126" and Breezewood.
Tim's referring to two-character (i.e. letter+number) routes, not the single-letter routes.
There's a Y1 in the Matamoras area.