The locomotive's ability to do its job is measured in "tractive power" or how many pounds it can pull. Granted assuming the boiler could potentially produce 500 horsepower, other factors need to be considered to pull a train. First factor is how much weight is on the drive wheels. Second factor is how many drive wheels are there. Third the size of the wheels also determine how much weight and how fast it can go.
If a locomotive is light in weight the drive wheel will just merely slip and go nowhere. More weight equals more friction on the track and less chance of spinning. Increasing the amount or sets of drive wheels creates more points of contact to the track. Lastly the size of the driver wheel determines the maximum speed. The larger diameter wheel enables the engine to go faster but cannot pull as much weight. Smaller drivers have greater torque and can pull heavy loads but not fast. Balancing the above factors enables the max horsepower to be used on the drive wheels. For this reason steam locomotives are not rated in horsepower but tractive power
To the left is the Minnetonka 1, the first steam locomotive of the Northern Pacific.
To the right is the 5144 class Z-8 Challenger.
Ron Nixon photos courtesy of Museum of the Rockies.
Please note the wheel arangement on the Minnetonka. There are no leading truck wheels or tailing wheels, just 4 drive wheels, making this a 0-4-0 wheel arrangement. The Challenger Z-8 4-6-6-4 class was one of the last manufactured steam locomotives for the Northern Pacific. The drive wheels are articulated, that is 3 sets of wheels (6 total) on its own independent pivoting truck, to allow for the purpose of negotiating turns. The articulated truck sets rotate in the turn so that lateral pressure on the inside of the track is reduced to prevent track from spreading.
Thanks to The Museum of the Rockies and help from the Northern Pacific Railway Historical Association a roster with pictures of Northern Pacific locomotives is on display. Click here to be transferred to the roster.
Click on any of the locomotive pictures below for more information.