The slug is basically a stripped-down DE6 locomotive. It has no operating controls, cab, or engine of it's own. Instead, it draws electricity from another Diesel-Electric locomotive to power it's own traction motors. Because the slug has no engine of it's own, it cannot produce any extra power or move by itself. The inside of the slug is mostly filled with concrete to give it more weight to increase traction.

The purpose of the slug is to increase the traction of another locomotive. The connected locomotive's power is divided across more axles. This helps to prevent wheelslip which allows the locomotive to climb hills and start heavy trains more easily. Additionally, the slug is equipped with it's own air compressor, which allows it to help refill the brake lines. This is very useful because the slug naturally allows for longer trains which require more compressor capacity.

In career mode, the slug is not obtained by licence. It is instead obtained like other crew vehicles, by purchasing a garage key from a specific shop, then going to and unlocking the garage. For the slug, it is in Steve's garage, which is located in the far south-east corner of the map, at the end of the long branch line near the Harbor. The key for the garage can be bought at the Harbor's shop.

To get the slug out of the shed, you can summon it to any location (on a train track of course) at any time by using the "Crew Vehicle" option on the comms radio. Doing this costs $5,000, and unlike fast traveling the cost is fixed and is not based on how far the slug must move to get to where you summon it.

Another option is driving a locomotive down the branch all the way to the garage, and then physically towing the slug to where you need it. This is potentially dangerous though, as the branch line features the steepest grade in the game, 5.5%. It also features several very sharp turns, with speed limits of only 10 KPH.

In sandbox mode, the slug is automatically unlocked, but only if you have previously unlocked it in a career session. It starts in it's shed. The "Crew Vehicle" option for the comms radio is disabled in sandbox mode, so you cannot summon it as you would in career. However, any amount of slugs can be spawned for free by using the "Vehicle Spawner" option on the radio.

Diesel-electric locomotives use a diesel engine to run an electricity generator. The produced electricity is directed to the traction motors, which turn the wheels. When you apply throttle on a DE locomotive, you produce power to move, and a side effect of this produces amperes.

Amperes are a complex topic in the electrical engineering world, but in this case the concept is rather simple. When you start producing electricity with the engine, the electricity starts trying to turn the motors. However, there will always be some amount of resistance to the wheels turning, which means that even though the motors are trying to get the locomotive up to full speed, they cannot.

This effect produces amperes. You can think of this as wasted electricity that tried to turn the motor but could not. Imagine a waterwheel that is running a machine. The machine takes a lot of force to turn. If you run a pipe with a massive amount of water flowing through it to the waterwheel, some will contribute to turning the wheel, but because the wheel is limited in capacity, most will overflow out of the buckets, bypassing the wheel. The water that spills out of the wheel is our amperage.

However, the "wasted" electricity is not really wasted. It still contributes to getting the train moving. Thinking about the waterwheel again, even though the water is overflowing around the wheel, every drop of it is still contributing to getting the wheel to turn faster. The same thing is true with the electricity. More energy flowing to the traction motor means more force that it is using to accelerate.

Now here we come to traction motor temperature. One of the basic laws of physics is that energy cannot be created or destroyed. However, it can change form. The energy that actually turns the motor simply becomes motion. However, as I explained, not all of the energy actually turns the motor. It produces amperes. So where does this energy go?

It produces heat. Lots of heat. Putting the petal to the metal on a diesel-electric locomotive causes a massive amount of electricity to be generated. Again, not all of the electricity actually turns the motor, and depending on how hard it is to get the engine to go faster, which could be changed by many factors, a massive amount of heat can be generated. The heat generation is tied to amperage. More amperes means more heat.

Simply put, all this means:
Difference between goal TM RPM (or throttle amount) and actual TM RPM = Amperage
Higher amperage = More heat generation

Dynamic braking on the DE6 does produce amperage, but the locomotive has purpose-built radiators to get rid of the heat produced, so there is no concern for overheating due to this.

Ok, so now that you understand amperage and the issues it causes for diesel-electric locomotives, I'll show you how the slug solves the problem.

Both the DE2 and DE6 are easily capable of producing enough power to overwhelm their traction motors. One thing to note about this is that TMs will shut down because of excessive amperage. If the amperage does too high, the TM breaker will be tripped. TMs can only fail completly when they reach critical temperatures. If the temperature gets too high, this will usually just trip the TM breaker, but it can also result in one of your TMs failing completely. The destroyed TM is expensive to repair and until you do will not function. Your engine power will end up being divided to all other axles, which can make it harder to manage wheelslip.

When a slug is attached to a locomotive, it's like adding a second waterwheel. There's still only one engine though, and we can imagine this as one pipe's water flow being split and used to turn both wheels. This results in it being far more difficult, if not impossible, to overwhelm the waterwheels.

However, this of course means that both waterwheels receive half the force. This is a good thing though, because with a train this reduction of force per axle prevents wheelslip. This also means that it solves the amperage problem, because half the electricity flowing to each axle means half the possible amperage, and half the amperage produced in any situation. This vastly reduces TM heat generation.

Additionally, the power input and collective power output is the same. Using the slug does not reduce the overall power output of your locomotive.

The slug is currently compatible with two locomotives: The DE2, and the DE6. Even though the DH4 seems like a potential candidate for powering the slug, it cannot. The BE2 is also not capable of using the slug because it lacks an MU cable. The E6, an electric locomotive planned to be added in the future, likely will be able to power the slug.

To connect a slug to a compatible locomotive, simply connect the slug's multiple unit cable to a locomotive you want it to assist (draw power from). The MU cable is fully functional, meaning that MU signals can be transmitted though it, allowing lash-ups like DE6 - Slug - DH4. It is possible to connect a DE locomotive to both ends of the slug, and the slug will draw power from both. This allows lash-ups like DE2 - Slug - DE2, and in this case both DE2s can work together to power the slug's traction motors.

Note that even though the slug has MU functionality, it does not have an independent brake system. This means that stopping the lash-up with independent brakes will be more difficult.

When using the slug with the DE2, the performance of the locomotive is effected. The DE2's tiny engine, which is only meant to power two TMs, is understandably effected by the slug being attached. Because of the rather large power distribution, the locomotive accelerates rather slowly. However, this means that the engine can be put into full throttle from a stop without issue in most cases, which makes shunting easier. Keep in mind that the large power requirements will need the use of higher throttle settings, which could drive up maintenance costs higher than expected. Using the slug with one DE2 is still more efficient than using two DE2s though, because despite the lower speed you are only running one engine.

Using the slug with a DE6 on both ends is the current META. The slug really helps to get the full potential of the DE6. When running the DE6 on it's own, it has trouble using all of it's pulling power due to wheelslip and TM overheating. Assisting it with the slug solves this problem.

Remember: The slug increases traction not power. Because the slug distributes the power over more axles, this reduces acceleration, but overall pulling power and top speed stays the same. A DE6 - slug combo at full power is similar to two DE6s on half power. However, you of course save fuel because you are only running one locomotive. This factor can make the $5,000 summoning cost worth it.

Note that the slug requires manual maintenance to keep it operational. Being personally owned by the player, it is not covered by the railroad's insurance. The slug carries and consumes sand, so it's important to remember to refill this.

Thanks for taking a look my guide. I hope it made sense and it will help as many confused engineers as it can.

If you have any questions that this guide did not answer, or you see something that I got wrong, please let me know!

Drive safe, and have fun in the valley. Enjoy!