Kinetic Energy[]
Diesel Locomotives weighs around 91,000 to 181,000 kg and their top speed are 125 miles per hour. The weight of the train will be 136,000 kg as the average, since it is more safe to use.
Kinetic Energy: 1/2 * 136,000 * 55.88^2 = 212,335,059.2 Joules, 212.3350592 Megajoules, 0.0507492971 Tons of TNT, Small Building level (9-A)
We can use the Low-End (91,000 kg) to High-End (181,000 kg) just in case, so the top speed of 125 mph - 55.88 m/s is still used. Joules has been converted to Tons of TNT.
Low-End: 0.0339572503 Tons of TNT, Small Building level (9-A)
High-End: 0.067541344 Tons of TNT, Small Building level (9-A)
Fictional characters who survived getting hit by a train will be 9-A if slammed into a wall. Trains accelerates at Tens of seconds to reach 60 mph and 125 mph is its full speed, you can argue that the average speed is 92.5 mph, so the train yields 116,274,678.418 Joules, 0.0277903151 Tons of TNT, Small Building level (9-A) at average.
A grizzly bear named "The Boss" survived being hit by the Diesel locomotive train, not only once, but twice, and yet he survived both encounters. Before we say grizzly bears are easily 9-B, I probably don't think so. "The Boss" is world's toughest bear, so this only upscale "The Boss". I don't think average grizzly bears can scale to him, but otherwise possibly.
The Boss weighs 295 to 317 kg [306 kg as average], so 1/2 * 306 kg * 55.88^2 m/s = 477.7538832 Kilojoules, Wall level (9-B)
Stopping the Diesel Locomotive Feat[]
This is for fictional characters who are shown to stop the Diesel Locomotive train (The feat will be 9-A since a fictional character stopped the Diesel Locomotive train that is going at its full speed and the Kinetic Energy was outscaled by a fictional character). After calculating the Kinetic Energy, we will now use the train's speed (125 mph - 55.88 m/s) and the fictional character's reaction time (Depends on the reaction time), but I will take 1 second as a placeholder. Whatever their reaction time is, use it to divide by the speed of the train:
Formula: Speed (m/s) / Time (in seconds) = Acceleration (m/s2)
Depends on the train's speed being used at the time since they don't always go at 125 mph (Since it is a top speed), they go to whatever their speed were calculated by someone (If it is not going at the top speed). I will use 125 mph - 55.88 m/s as a placeholder (Unless a fictional character stopped a diesel locomotive train that goes to its full speed, then it can be used).
55.88 / 1 = 55.88 m/s2
After getting whatever the acceleration you got, you can now use the mass for the train (Depends on the weight you are using for the context. Use the average (136,000 kg) if you want to use something safer)
Formula: Mass (Kg) * Acceleration (m/s2) = Force (Newtons-Force)
The acceleration will also be variable to yours if the train's weight/speed you are using are different.
Low-End: 91,000 * 55.88 = 5,085,080 Newtons-Force, 518,533.85203 Kilograms-Force, Class K
Mid-End: 136,000 * 55.88 = 7,599,680 Newtons-Force, 774,951.69094 Kilograms-Force, Class K
High-End: 181,000 * 55.88 = 10,114,280 Newtons-Force, 1,031,369.5299 Kilograms-Force, Class M
This can be used if the train is going at the full speed. Otherwise you have to calculate its variable speed to use for the force. The weight will also be variable, but using the average (Mid-End) is safe for now.