There are 5 scenarios from the base route DLC that you must complete, start to finish, for this achievement:
Rise & Shine (Estimated length = 10 min)
Freight Exchange (Estimated length = 45 min)
Returning Empty (Estimated length = 45 min)
Storm Trail (Estimated length = 60 min)
Heavy Metal (Estimated length = 50 min)
I would advise doing all of these through the Journey chapters for this route if you don't already have the 10 Journey Chapters complete achievement, as they will not count as completed for the Journey if you finish them under the Explore tab.
NOTE: I had the BR 155 locomotive DLC installed, which contained 3 additional scenarios. This achievement did not pop until I had completed all of those as well (as of October 24, 2020).
UPDATE FEB 2021: Currently the Freight Exchange scenario is not completable due to a signal issue. However, TA user Blissed Away has found a potential workaroundFreight Exchange red signal workaround for Train Sim World 2
There is a finicky way of getting the next objective and path to trigger. It involves going very slowly through 3 danger signals (0.0km) at the start. This doesn't trigger a signal passed at danger message when done at very low speeds. You need to use the direct brake and I advise using the controller triggers for this. Going past these at 0.1km may fail the scenario. I will explain this further but first I will make a few points...
This method doesn't always work. Sometimes a freight train ends up stopping at a red near you. I don't know why but it should normally go past even in this scenario's current state. When it is stopped here, your signals after coupling will be affected. It could be because of saving the game and resuming it (this is an issue in TSW where manually saving can cause more stuck signals). Another thing could be your position and time taken. I was doing this slowly so by the time I went by the 3rd red no more freight trains were going past. There is a passenger train that is stopped up ahead - this one will only move when you stop at that objective marker after getting past those red signals. A train near the end activates after reaching another couple objective. If it doesn't move, your signals may be affected again. I have tried this scenario a few times recently and 2 of 3 times I was able to complete it. The time it didn't work was when I manually saved and resumed the saved game (more stuck signals).
How to do it:
First get up close to the red signal around 3m (4m to play it safe) according to HUD. You need a little room in case you accelerate too fast. Now use direct brakes only (switch with X button). Apply these. Set throttle to 1% (very sensitive). Release brakes and move back to hold aiming for 0.1 on the BC (0.2 if the speed goes above 0.0km) on the BC HUD. The locomotive will move very slowly continuously as you can see in external view with the wheels. If not try a little more throttle and brakes - you have to find that sweet spot. Keeping it at 0.0km when going through red is the trickiest bit. Any higher may cause a SPAD message. Once past the first red and the distance to next signal resets, proceed as normal and carefully stop before the second red signal. Do the same steps for the final red signal.
Stop at the next red and the objective/path will trigger. Request permission when it says to and couple. Ensure you release the direct brake. Hopefully, the rest of the scenario goes well.
Video of the last red signal being passed:
This method will not work for Cement Circle on TSW 2020 as track points will not be set (in Freight Exchange we are going straight ahead and the points are already set).