The 1870 kml files show the Lackawanna and Bloomsburg railway following the tracks of the Pennsylvania railroad, south and east of the Susquehanna River. The L&B was absorbed by the DL&W, which runs along the north and west side of the river, as per the USGS quads. Taking this route allows Twain the take the Catawissa, Williamsport and Elmira to Williamsport. This line may be misnamed. It's likely that it is actually the Catawissa, Williamsport and Erie, which became the Catawissa railroad which eventually merged into the Reading RR.