The picture in Railway Magazine July 1975, of the Sunday 6 April 1975 up train, only has passenger coaches and looks to be 4 or 5 coaches.
I became quite familiar, off and on, with the Belfast Boat Express in its final few years with steam - although way too young to be in the 20-55 Club, or even by the lineside at that time of day.
I contributed to an "Interest in the Railways - where did it start?" thread with
this post from 2016. . .
Springs Branch said:
As a youngster during the 1960s, in the summer school holidays I usually went to stay with an uncle and aunt who had a house about half-a-mile across open fields from the railway line between Lostock Jn and Horwich. Most nights at bedtime, the routine was I would sit by the dining room window to watch the "Boat Train" passing, then off to bed with no further arguments. This was the Manchester Victoria - Heysham Belfast Boat Express, which became one of the last steam-hauled passenger expresses on BR.
I didn't know much about railways, but seeing that train each evening in the summer twilight, steaming hard uphill from Lostock Jn, followed by its train of warmly-lit carriages made an impression which must have primed my interest.
Memory is fallible, but I'm sure the BBE loaded to more than 4 or 5 coaches then, and presumably also into its first few years with diesels.
As I understand it, the demise of the BBE - along with the Sealink ferry to Belfast,
The Ulster Express from Euston and other connecting boat trains from Crewe & Birmingham, or Leeds via Wennington - came about because of the 1970s Troubles. The Troubles weren't necessarily a thing to discourage visitors and tourists from Northern Ireland in 1968, but by 1975 they very much were.
The decline and imminent fall of BR's Heysham - Belfast ferry service was debated in the House of Commons on 6 March 1975 . . .
Hansard 6 May 1975 said:
Mr. Robert J. Bradford
(Belfast, South)
. . . . First, there has been a calculated commitment to the removal of this sea link over a number of years. In other words, a hole has deliberately been made in the base of a bucket and that bucket has been discarded because it cannot hold water. The express parcels service ceased using the Heysham-Belfast service, and its custom was transferred to the Stranraer-Larne service in October 1973. In that year, however, the Belfast-Heysham service was noted for its excellent handling of that parcels service. Why then was the custom transferred?
Included in the debate were several contributions from the redoubtable MP for Antrim North, the Rev. Ian Paisley, including . . .
Hansard 6 May 1975 said:
Rev. Ian Paisley
(Antrim, North)
Is my hon. Friend aware that the transport users' consultative committee for the Western Region laid evidence before the inquiry that the brochure printed by British Rail had already omitted the Belfast-Heysham link, so that tourists wanting to use the route this year cannot even know that it exists?
So it's plausible the dieselised BBE began as a train of normal 'express' length in 1968, requiring a Class 40, but custom had declined by 1975 such that 4 or 5 coaches and a Type 2 would do.