Richard, I just recovered these notes from the development thread of the "Monolith Cement" session (if you feel lucky with Google Translate, here are 17 pages in Italian).
11.39 - Coupled to the caboose and brown hoppers at the rear of the train just arrived on track #3
11.49 - Brown hoppers delivered on track #4 [+100 points]
11.59 - Locomotives and caboose coupled to the departing boxcars on track 5
12.14 - Boxcars and caboose coupled to the freight train stopped on track #3 [+100 points]
12.18 - Starting to move to the head of the brown hoppers on track #4
12.25 - Locomotives coupled to the head of the brown hoppers on track #4; departure towards the quarry
13.01 - First 10 brown hoppers left on quarry track #1 [+50 points]
13.04 - Second 10 brown hoppers left on quarry track #2 [+50 points]
13.05 - Departure from the quarry with the two GP-9s running light to the cement plant
13.23 - Locomotives coupled to the white hoppers at the cement plant [+100 points]; departure to Laramie
13.34 - Arrival at Laramie: decoupled from the white hoppers on track #6
13.39 - Locomotives coupled to the caboose assigned to the train being assembled on track #6
13.45 - Caboose coupled to the white hoppers on track #6
13.47 - Caboose and white hoppers pushed to couple with boxcars on track #6; departing train assembled; steam loco couples to the train and departs heading west [+100 points]
13.53 - End of session
As you can see, you have to push some boxcars on track #3 in the initial stages of the session (highlighted in red), when you couple them to the train from which you took the brown hoppers that you will have to deliver to the quarry at a later stage. You must not move any boxcar during the final stage of the session.
If you loaded a saved game, it is highly likely some "flag" variable (such as "GP9", which is shown in the HUD) has been resetted when you loaded the game.
Such variables are used to monitor the session progress (e.g. by having an instruction window displayed when you pass over a TM, but only at a specific time). If you are - by way of example - at stage "3" and the variable value is "1" the instruction windows displayed will be the wrong ones and the session will not work properly, as other rules depending on such variables will not trip at the proper time.