Cylinders On Top Of C&NW GP7 ??

Those are the air reservoirs, they are on the top because of the larger combined fuel/water tank underneath.  The water portion is the supply for the steam generator for passenger service.

Rusty

Many RR had them, Including the PRR #8553

That is commonly referred to as the GP7-TT the "TT" stands for Torpedo Tubes

It seems that most GP's with torpedo tube air reservoirs didn't have dynamic brakes.  Most, but not all:

Rusty

When I first saw that C&NW unit, I couldn't figure out what was going on with the dynamic brake.

Upon learning that they were air reservoirs, I did some reading, and made the assumption that they were only on non-DB units.

I see that Rusty knows better.

Second generation units with DB's had them as well.  Frisco had GP35's with DB's and extra large ruel tanks, so the air tanks were mounted on the roof on either side of the DB fan.

Stuart

As I recall from reading, one of the criticisms of the original GP7 design was a somewhat smallish fuel tank capacity. Originally the air tank was under the frame. By moving the air tanks to the roof it was possible to have a larger fuel tank AND larger air tanks. win win.

Well, the GP7 had a fuel capacity of 1600 gallons vs. the 1200 gallons of the F7.

Rusty

Not really. The need for a "larger fuel tank" was for passing service units equipped with a steam boiler for train line steam. Thus, such units require a combination fuel & boiler water tank. The main air reservoirs then were relocated to the top of the long hood.

Another thing to note on the CNW unit is the "bulge" at the end of the long hood. That was for the HEP generator equipment for the passenger GP7's. They also had a couple of GP9's similarly equipped. MTH had an early Premier GP9 with the roof tanks, but equipped the unit with the freight fuel tank with another set of tanks and minus the generator equipment.

Here's a better shot of a GP9r, which kept the hood modifications even though it had been some time since it had seen any passenger duty.

...or maybe not.

C&NW's 4320 at Marshalltown, Iowa. March 1981. My picture.