The Cars aren't required to make it profitable; they actually make it worse. Broken down by preferences (all decimals rounded to 3 places):
1. If you prioritize FPs, you choose the Evergreen Express, which is 8FP/21sq for an efficiency of .381 FP/sq/day. If you attach a Sleeping Car, you now have 10FP/27sq for an efficiency of .370 FP/sq/day. 2 Sleeping Cars? 12FP/33sq for an efficiency of .364 FP/sq/day. 10 Sleeping Cars? 28FP/81sq for an efficiency of .346 FP/sq/day. And so on.
2. If you prioritize goods, you choose the Sleighride Express, which is 20goods/21sq for an efficiency of .952 goods/sq/day. Attach a Freight Car, it becomes 25 goods/27sq, .926 goods/sq/day. Attach 10 Freight Cars, you get 70goods/81sq, efficiency .864 goods/sq/day
3. If you prioritize attack boost (my people
...), you choose the Charcoal Express, which is 20%/21sq for an efficiency of .952 atk%/sq. Attach a Dining Car and it becomes 24%/27sq, making it .889 atk%/sq. Attach 10 Dining Cars, 60atk%/81sq, efficiency of a paltry .741 atk%/sq.
Now, for the coup de grace-it's worth pointing out that each of the Cars only boosts ONE of the three main resources (FPs, goods, atk boost%), so the efficiency of the other two are going to drop very steeply as you add Cars. So let's say I do the last option. Not only does the Charcoal Express go down in efficiency by 22.164%
of its specialty, it also means that the Charcoal Express is now producing 5FP/81sq for an efficiency of .062% as opposed to the original .238 FP/sq/day, as well as 15 goods/81sq for .185 goods/sq/day as opposed to the original 15/21 .714 goods/sq/day. Drops of 383% and 386% respectively. It should be a trade-off (i.e. "is it worth the reduction in efficiency of production for [non-preferred resource A] and [non-preferred resource B] in order to increase the efficiency of [preferred resource C])?" But
they're so badly balanced that they even hurt the PREFERRED resource! And they want you to spend real-world money adding these Cars!!
Note: it's been a long week so if I've made an error in the math I apologize in advance...