alt Hacker NewsI depends on the background. I'm 2 years into solo developing a game and all programming, artwork and animation is my own. I had to invest into quite some learning to make it possible, but I figured it's a worthwhile investment. I do work with a composer, though.
Point being, it depends on which skills you bring to the table, which ones you are willing to learn and which ones are worth collaborating on.
I still think the term Solo-developer is justified in any case. The one who soley carries the burden of bringing the game from idea to the finish line is the solo developer, IMHO.
If a project is far more work than one person can do (as this train simulator obviously is), the term solo developer is no more justified than it would have been if Steve Jobs claimed to have solo-developed the iPhone, even if he could have justified saying he "brought the iPhone from idea to the finish line."