There's an account of it in Staufer's book New Haven Power and probably the NHRHTA Shoreliner maybe had a article on it.
Two conventional coaches (the power unit was probably a combine) were given rounded rooflines to mimic streamlining and one was fitted with an operating cab and fake Zephyr/Comet style end treatment...at one end. A truck was removed from the cab car and a special power truck with the Besler steam engine was fitted.
The Besler operated for several years into early 1940s almost always between Bridgeport and Waterbury. It was a one of a kind set (the NH had a number of one of a kind things in the roundhouse and on the road over the years) so during World War II, the cars were converted back to conventional locomotive hauled equipment.
If I can get up into my attic soon, I'll see if I can find an employee timetable or two. The Besler (along with the Comet, Shoreliners, Little Shoreliners and for a time gas-electrics) was shown when regularly assigned to particular trains.