I've seen this played differently in two different play-throughs - so it may be good to get clarification.The second 1-Engine is removed from the game. The rulebook says "setup as a 2-player game", the other 1-Engine will be then on Emil's plateau, which doesn't exist, so it's out of the game. It can't be available for purchase, since that will not be a standard 2-player game.
In the 2 player and in the solo game you remove two of each engine type - so two of each type remain. Of the 1-engines, you put one on your player board as a start item.
Q: Now in solo, does the second 1-Engine start as part of the locomotive offer or did Emil collect it already and it is not available for purchase?
In a regular 2-player game, your opponents 1-engine would start on his player board. But the setup does not mention to set it aside. Emil also does not have a player board. However, the rules do state that Emil collects engines....
I play it as set-aside since this resembles the regular 2-player game most. Still curious what you think.
Source: interpreting the rulebook as logically as possible