ROCK ISLAND GP7 Air Brakes

Some Air Brake Systems Background

When MNTX 559 was built as CRIP 1223 in 1951, EMD offered the GP-7 with a choice of air brakes: 6-BL, the tried-and-true steam locomotive air brake adapted for diesel switcher use, or 24-RL, a new post-war design for diesel road locomotives. Both allowed grouping diesels into one multiple-unit locomotive, "if" all had the same style air brake. Mixing was possible, but problematic. "HSC" is standard 24-RL with electric control added to shorten the stopping distance of high-speed passenger trains (allowing faster schedules.) It was not very reliable, and disappeared quickly. The 26-L design appeared in the mid-fifties, and today is the only design in common use. Upgrading was common in capital rebuilds. An optional 26-L feature is the ability to be controlled from engines with any air brake type. When 6-BL and 24-RL were still common this was useful, but today it has been stripped from most locomotives to reduce maintenance costs.

The following text is from RW Haave describing the original air brake system for MNTX 559, nee CRIP 1223

As built for the Rock Island the unit had 6-BL (one pipe) system. When rebuilt in the '70s this unit received 26L air equipment with a slighlty different MU2A valve that was set for:

I know, you are thinking that 26L is a 2 pipe system, and normally it is. The RI ran theirs as a 1 pipe system so as to lead or trail MU with all the 6-BL GP7 & GP9's. In earlier years the RI modified their 24RL and HSC units with an extra pipe they called "MU pipe", this enabled the 6-BL units (one pipe) to trail the 2 pipe units. As built a 1 pipe unit can not lead a 2 pipe unit, as the second pipe is the actuating pipe which when pressurized "Bails off" an automatic brake application. Now a photo might someday show up with a RI GP7 leading a covered wagon (F-unit).
How did they do it? You coupled the front sander pipe of the GP7 to the actuating pipe of the F-unit. When ever you needed to "Bail off" the engine brakes on the second unit you turned on the front sanders. Not FRA (ICC way back) legal, but it worked, was called the "Corn field hook up" as I recall. The 10 ex-DRGW GP7s were 24RL. Note: RI purchased 10 ex-DRGW GP7s in November 1972 and these units would not MU with standard RI units due to incompatible brake systems.

After your unit went to the CNW it proably got a normal MU2A valve. The Soo Line modified all of their 6BL units so they could lead or trail the 24RL units.

I was in engine service on the RI Illinois Divn from 1966 to 1976, spent one winter loaned out to Esterville as engr. Then I went to BN at Edgemont. When I regained my sanity I hired out on the Soo Line in 1978.

Dick Haave, aka rrboomer
Portage, WI