The new Cascade Tunnel was opened on January 12, 1929. The original was opened on December 20, 1900 and was only 2.6 miles long. So I am pretty sure the Boston tunnel was the longest prior to 1929. Read the Wiki, great reading...
https://en.wikipedia.org/wiki/Cascade_Tunnel
They used Z-1 Locos... See here;
https://en.wikipedia.org/wiki/Great_Northern_Z-1
That is correct, my error. The Great Northern Z-1s are similar to the New Haven box motors and of course similar to the ones used on the B&M until 1946 when they dieselized. The B&M motors, their 5000 series, ran for nearly 40 years before they were scrapped and were built in conjunction with the New Haven with help from GE and Westinghouse.
Here's an article on the electric service.
https://reference.insulators.info/publications/view/?id=11903
The B&M also used 11KV and then stepped it down inside the locomotives. They, however, used a catenary system similar to that found on the New Haven, but a lot shorter and not as fancy. I'm sure the New Haven had a bit of help with this since they were both owned my J.P. Morgan at the time and partnered with a lot of projects. The plan, before J.P. Morgan sucked the funds out of the companies, was to electrify the B&M Connecticut River Line. The Fitchburg line from Williamstown, MA to Deerfield, and the Eastern RR. None of the expansion plans occurred, but the B&M gained a 4-track bridge in Lynn, MA where the Eastern was to be expanded to a 4-track mainline that was never to happen.
Mike, aka Steamboateng. has been working on a Hoosac Tunnel project using actual DEM data and 1944 (or thereabouts) topo maps. I've taken his current route as is, chopped down stuff outside of the area I'm interested in, and I've been slowly rebuilding the Hoosac Tunnel & Wilmington, a connecting short line better known as the Hoot Toot & Whistle, or Hold Tight and Worry. The HT&T disappeared in the 1970s after New England Power Co., now National Grid, flooded their ROW just below Madison, MA with the Bear Swamp power tunnel. The flooding completely changed the river level and wiped out their ROW. The railroad was too poor by then to rebuild. Today parts of the ROW are now paths, and lots are gone. Tracing the route on maps is nearly impossible, but with historic photos, approximation, and a lot of artistic license, my aim is to have this connecting short line come to life again as a tourist route. This unique route has a history all in its own as a 3-foot gauge route built to haul lumber with connections to further routes in the region, and this too could link to yet another project all in its own as a what if scenario with a lot of artistic license applied. Using DRG&W, and old BRB&L locos and coaches, plus the plethora of lumber cars built to that gauge, a lot can be done. Hmm... this has me thinking on this!