Why does the UK not have B units?

[MattRat]

For those that don't know, in the USA, locos can designated as an A or B unit. An A unit is what we already know of as a standard loco, but a B unit is essentially a loco without a cab, attached to a A unit which provides said cab, and the B unit provides extra power when needed, without needing extra crew.

Considering the benefits of such a design, turning a small unit built for light jobs or even shunting, into a mainline powerhouse capable of the heaviest loads, plus the flexibility of swapping in different powertrains like battery or even pantograph B units (modern software should be able to handle switching between power sources), why hasn't the UK built any locos with B unit capabilities yet?

 

[edwin_m]

Our trains are rarely long enough to require double heading, so little point in buying a locomotive that could only be used in such a formation. If any were used, they would have to be turned as a pair at the end of the journey to end up with the cab facing forward for the return, rather than just running round a pair of double-cab locomotives or coupling them to a later train. Triple heading with a B in the middle would avoid that problem, but I can only think of one flow ever that required three locomotives (Llanwern ore back in the 70s.).

This is less of an issue in America where use of three or more locomotives is common, and even if multiple locomotives need turning this is relatively less significant in proportion to the much longer journeys they will have made. Even so I don't believe America has built B units for many years, so evidently the flexibility and production economies of scale of having a cab on everything are worth the extra cost of having some cabs that might be used only occasionally.

 

[30907]

There were the Master-and-slave 08 conversions (13s?) at Tinsley.
Other than that, the only locos that ran predominantly paired were the 20s, and they are of course single-cab.

 

[Gloster]

And it is a long time since it was necessary for the second loco to have a driver. One driver can control two, or possibly more, locos.

 

[61653 HTAFC]

I'm a bit surprised that double-ended units are so rare in the US, especially diesels. I wouldn't expect them to be the majority of course, but it's surprising that so few were ever ordered given how useful they'd be for places that don't have space for a turntable or wye. In particular I'd have thought "short lines" would be able to take advantage of the flexibility. Though I suppose US locos are able to be a bit more generous with cab sizes, so can make them higher or wider (or both) than the engine compartment, which removes the need to turn the loco at all.

 

[MarkyT]

In the USA, steam locos had been getting ever larger with behemoths like Big Boys being the final outcome. This was primarily to avoid multiple crews for double heading which would have been a large expense. Cue post-WW2 diesel salesmen who offered their multiple working capabilities, so you could lash up as many units as you liked while only requiring one crew. The early diesel locos, both A & B types, were not very powerful individually, but the ability to couple together and control as many units as you needed for a particular duty was an operator's dream, and steam's final nail in the coffin during the 1950s and 60s.

 

[43096]

I'm a bit surprised that double-ended units are so rare in the US, especially diesels. I wouldn't expect them to be the majority of course, but it's surprising that so few were ever ordered given how useful they'd be for places that don't have space for a turntable or wye. In particular I'd have thought "short lines" would be able to take advantage of the flexibility.

They could, but the short lines don't buy many (any?) new locos. New locos are usually the domain of the big Class 1 carriers, certainly in larger quantities and they don't want dual-cabbed diesels (electrics have often been different). Cabs cost money, so why have two when you only need one - it's the same logic as all the fixed formation 10 and 12 cars sets we now have here on suburban duties.

 

[Trackman]

Versatilely I guess UK wise, a B loco would be a dead one without the availability of a A loco.

There were the Master-and-slave 08 conversions (13s?) at Tinsley.

I thought they were joined at the hip so to speak, so couldn't be separated as the master would not work. I guess...

 

[MarkyT]

Thought I'd just mention yet another variety of freight loco used in the USA, a 'slug' which has traction motors but no prime mover installed, getting it's power from the engine of a connected 'mother' locomotive. Sometimes these have a cab but many are cabless just like B units, and many have a long low hood as they don't have a large engine generator set inside and all its ancillaries to cover. Good candidates to pack with traction batteries in a hybrid configuration as with no big diesel and fuel tank they are likely to need ballasting anyway to reach desirable adhesive weight.

 

[MattRat]

Other than that, the only locos that ran predominantly paired were the 20s, and they are of course single-cab.

And it is a long time since it was necessary for the second loco to have a driver. One driver can control two, or possibly more, locos.

Versatilely I guess UK wise, a B loco would be a dead one without the availability of a A loco.

So basically, the UK sees the extra cost of every unit having a cab as worth it.

 

[Irascible]

This is less of an issue in America where use of three or more locomotives is common, and even if multiple locomotives need turning this is relatively less significant in proportion to the much longer journeys they will have made. Even so I don't believe America has built B units for many years, so evidently the flexibility and production economies of scale of having a cab on everything are worth the extra cost of having some cabs that might be used only occasionally.

Should mention that generally units are turned on a Y ( wye? ) rather than a turntable, so it doesn't really matter how many of them there were. The B unit died with the coming of the GP idea ( narrow body ), it didn't really last long.

Fairly sure they still turn units now, or at least shuffle them so there's a short end at the front. Their passenger diesels still seem to all have one cab.

 

[Dai Corner]

We do have the Stadler FLIRTs with their 'powerpacks', though they only carry engines and generators not traction motors.

 

[ac6000cw]

In historical terms, it's worth remembering that the original EMD 'FT' demonstrator of 1939 was, in concept, a single double-ended 5400hp 'locomotive' made up from 'A' units with cab plus cabless 'B' (booster) units in an A-B-B-A configuration, with semi-permanent bar couplings between units - so no need to turn it. It was the customer railroads who decided they wanted normal couplings between units to give them more flexibility.

The B unit died with the coming of the GP idea ( narrow body ), it didn't really last long.

There were cabless GP type 'B' units built (although very few in relation to locos with a cab), the last ones were 23 GP60Bs in the mid to late 1980s for the Santa Fe, which are still in service with BNSF. There were even 30 monster 5000hp cabless dual-engined DD35 locos built in 1963-1964 for UP and SP.

Should mention that generally units are turned on a Y ( wye? ) rather than a turntable, so it doesn't really matter how many of them there were.

There are still some turntables in use, but not many. As you say, a 'wye' (triangular track layout) is the common way of turning equipment. There are also 'balloon' tracks (like the Ely West Curve arrangement) in some places for the same purpose.

I did once see a 'long hood forward' lead loco on a freight turned around 'on the road' using the triangular mainline junction at Point of Rocks, Maryland (train stopped on mainline, loco uncoupled then run around all sides of the junction, coupled back on and train continued on its way).

Fairly sure they still turn units now, or at least shuffle them so there's a short end at the front.

They do - commonly mainline freights have most or all locos facing forwards (makes it easy to shuffle them around 'on the road' if a problem arises with the lead loco). Local freights usually have a 'cab at each end of the loco consist' setup as they often do out-and-back trips from a yard and to make freight car switching (shunting) along the way easier. Master and Slug sets (as mentioned by MarkyT in post #9) are also normally coupled back-to-back for the same reason.

Their passenger diesels still seem to all have one cab.

Yes (and always have had, AFAIK, right back to the 1930s power cars for the early 'streamliner' diesel trains). In contrast, the passenger electrics have generally been dual-cab (and still are).

 

[DelW]

Should mention that generally units are turned on a Y ( wye? ) rather than a turntable, so it doesn't really matter how many of them there were. The B unit died with the coming of the GP idea ( narrow body ), it didn't really last long.

Most of what I was just writing has been overtaken by @ac6000cw in his very comprehensive post above , but just to add some details of the B units built in the early days of road switchers, they included:
GP7B: 5 for ATSF
GP9B: 165 for Pennsylvania and UP
GP30B: 40 for UP
SD24B: 45 for UP
Chronologically the GP30Bs were the last of those, built in 1963.

(Edited to add source: The Second Diesel Spotter's Guide by Jerry A. Pinkepank, 1973)

 

[ac6000cw]

Most of what I was just writing has been overtaken by @ac6000cw in his very comprehensive post above , but just to add some details of the B units built in the early days of road switchers, they included:
GP7B: 5 for ATSF
GP9B: 165 for Pennsylvania and UP
GP30B: 40 for UP
SD24B: 45 for UP
Chronologically the GP30Bs were the last of those, built in 1963.

(Edited to add source: The Second Diesel Spotter's Guide by Jerry A. Pinkepank, 1973)GE B

To add some non-EMD units, according to Wikipedia, BN bought 120 cabless GE B30-7A(B) in the late 70s/early 80s, plus a few 1950s era Baldwin B-unit road-switchers. This is Wikipedia's 'world' list of B-units (diesels and electrics) - https://en.wikipedia.org/wiki/B–unit#List_of_B_unit_locomotive_models

There were some instances over the years when locos were rebuilt into cabless units after cabs got wrecked in accidents.