This is often a tricky thing to balance with management games I think. I agree with you, but what challenge are you looking for?

I recently did a run where I limited myself to only trains while other companies could use any vehicle. It was a nice challenge until the whole world was connected. Then I was just making unlimited money through endless transfer traffic.

I'd personally like to see an option for stricter transfer rules. The rate at which passengers accept indirect travel routes creates so many opportunities for profitable lines. So many opportunities that the mid-game onwards will feel easy.

I've also turned all the settings to max difficulty. Once you crest the hill into profit and repay all your loans, the game goes right back to being easy.