That was the original Reading & Columbia Railroad, later absorbed by the Reading Co. After the Reading was merged into Conrail, the R&C branch was operated by Conrail, then by the shortline Middletown & Hummelstown RR. They stopped operations over a decade ago, and now the line was recently purchased by a local company who is planning to re-start service. (after the line was nearly lost to a pathetic, lying rail-trail group.)
The original Reading & Columbia ran between its namesake towns, but now the line is in several pieces, with some being operated by shortlines, some by NS, and other portions being in rail-trail form.
The portion you saw under Rt. 30 starts in downtown Columbia where it interchanged w/ Conrail's (ex-PRR) Columbia branch. It then ends just beyond where you saw it at Route 30. (total length of about 2.5 miles)