Good explanation. I would just point out that in the 20 minutes rule you also have to consider waiting time. So it would be: walking time from oil to truck station + waiting time at oil station + transportation time by truck + walking from truck to train station + waiting time at train station + transportation by train + walking time from train station to demanding inustries.
I am not sure about how to calculate waiting time (I think it’s the displayed frequency, maybe /2 to get an average) and transportation time (I think you can estimate it by frequency * number of vehicles on the line /2).