It was double track:

The last regularly scheduled passenger train passed through the tunnel in 1958. As of 2012, the tunnel is part of Pan Am Railways, formerly the Guilford Rail System, with trains operated by subsidiary Springfield Terminal Railway, and the property and infrastructure owned by subsidiary Boston and Maine Corporation. The route is currently used to transport freight, but sees rare excursion trains for some special occasions. It was converted to single track in 1957. Clearances were increased in 1997 and 2007, the former by lowering the track, the latter by grinding 15 inches off the roof,[8] allowing trailer on flat car (TOFC) and tri-level automobile carriers to pass. In March 2012, the Federal Railroad Administration awarded a $2 million grant to the Massachusetts Department of Transportation for preliminary engineering on further increasing clearance in the tunnel to allow double stack container trains to use the tunnel.[9]

Currently, as many as 12 trains per day operate through the tunnel, without a schedule, including cooperative traffic from Norfolk Southern as part of the Patriot Corridor.