My understanding of a modern, conventional diesel electric railroad locomotive is that there are three driving components. My descriptions might be too simple but I think are basically correct.
(A) The prime mover. This is a diesel fuelled internal combustion engine providing mechanical [driveshaft] power.
(B) An electrical generator physically connected to the prime mover. The driveshaft of the prime mover rotates and rotates the electrical generator.
(C) Traction motors. An electrical motor physically driving the locomotive wheels (using gears I suppose). The traction motors (one motor per driving wheelset) are electrically connected to the generator.
QUESTION 1: Am I correct so far?
QUESTION 2: In normal operation, can the locomotive engineer/driver physically disconnect the generator from the prime mover?
QUESTION 3: In normal operation, can the locomotive engineer/driver electrically disconnect the traction motors from the generator?
I realize that both these things would cause the locomotive to become a free rolling unit. But that is what I really want to know.
QUESTION 4: Can the engineer/driver make an operational locomotive become free rolling while in otherwise normal operation?
Let’s please ignore all aspects of train and locomotive braking. I just want to know if this can technically be done without invoking lockouts and failsafes for dangerous operation.
It must be possible to disconnect the traction motors from the generator, because most diesel locomotives have the capability of raising a contact bar to a catenary wire, and running the loco in pure electric mode. If the generator were still connected when you did that, it’d spin like crazy.
But that transition is usually done over the span of several minutes, while the train is stopped. I don’t know if it can be done at a flip of a switch in the cab.
I don’t think in general physical disconnection is possible.
Electrical disconnection must be an option. The motors run reversed in order to reverse direction. Further, diesel electrics often are set up for dynamic braking, where the motors are connected to large resistors (with huge fans for cooling) allowing the locomotive to supply significant long term braking force to the train. Very valuable on long descents.
There are a whole range of options about how the electric system operates. The system can be AC or DC, with modern AC systems using variable frequency inverters to provide fine grained control of AC motors.
Traction motors use field coils, and these need energising as well. Traction motors are not a simple connection to the power supply. Control of current through the field coils is an important part of locomotive control.
Overall, there is a complicated (and very high power handling) system involved, and the ability to disconnect generator/alternator from the traction motors is least of the complexity.
If the generator (alternator) was electrically connected to the catenary, it might spin. But that’s a reason for electrical disconnection, not mechanical disconnection.
In the UK many commuter trains are DMUs (diesel multiple unit) that don’t need a locomotive. Where the lines are completely electrified there will be EMUs like this one.
Okay, looks like no physical prime mover / generator disconnection but almost certainly the ability to electrically disconnect the traction motors. Now let’s focus on question 4.
QUESTION 4: Can the engineer/driver make an operational locomotive become free rolling while in otherwise normal operation?
The commuter train I used to take had two lines, one electrified, one not, coming together to an electrified portion going downtown. The one line was pure electric (25 KV, AC) and the other was diesel electric. The transition was made at my stop and I watched it carefully. It took well under a minute to shut down the diesel and raise the pantograph on the incoming train and also under a minute to start the diesel and lower the pantograph on the outgoing one. In both cases it was done only when stopped at the station. And entirely from the driving cab. They consisted mostly of two unit pairs, one driving car (with its own pantograph) and one drone. But it was all done from one driving cab. Whether the diesel engine was actually stopped or just idled I cannot say. But they went into a tunnel that was considered too poorly ventilated to allow diesel operation at power. Although I did once take a diesel-powered train during a province-wide power failure.
I wouldn’t say “most” diesel locomotives have this capability; what you (and @Hari_Seldon) describe is called an “electro-diesel locomotive,” and they are a fairly niche market segment. In the U.S., they exist pretty much entirely for passenger trains that run into Grand Central Station or Penn Station in New York City, where they are not permitted to run on diesel power.
Most diesel locomotives are only capable of powering their traction motors via the on-board diesel engines, and lack the equipment to tap into an overhead power wire or a powered “third rail.”
Without a clear cite, but I would say yes. Just removing the excitation current from the field coils will yield a free rolling loco. But to imagine that there is no “off” is very difficult to imagine. Especially as directly jumping from one of the possible “on” connections to another would be akin to selecting a random gear-change in a car (where reverse is one of the possible gears.) Moreover, a loco with a variable frequency inverter can also be simply turn off the inverter. In reality you are talking about very large and dangerous voltages and currents that need to be handled. The systems are not just a few switches. Getting something wrong will likely lead to catastrophic and eye wateringly expensive damage.
Locos are often attached to trains for delivery runs and powered down. They need to be able to free roll.
You might be interested in the description of a locomotive at the above link. It implies that the diesel was shut down when necessary to avoid exhaust fumes.
Sometimes the Diesel Electric is along as a backup system. When the Union Pacific Big Boy made it’s tour recently, last week or so, there was a Diesel Electric right behind in case there were mechanical issues. It might also have been supplied electric power to the cars hooked on behind as well.