Board Game Arena

Wondering if I can find a good player base on this game or if I should go to the Russian Railroads game (and lose all the ultimate content )

I have open game ULTIMATE RAILROADS on Arena. But it's impossible to enter a table - I have always ERROR 500

You could go to All Games and sort by popularity. That will give you a sense of their relative activity, though it does not provide absolute numbers (there used to be a table with a count of all active games, but I think it's no longer publicly available).

IIRC, Russian Railroads is slightly more active than Ultimate Railroads but they were relatively close.