Played TTD for hundreds of hours as a kid. The soundtrack and ambient sound effects are still so clear in my head - distant sounds of industry and vans and busses and trains just doing their thing. And then occasional updates popping up. It had a really nice flow to it. Very addictive. I'm fairly sure I spent at least one 'sick day' off school playing TTD instead.
I've tried OpenTTD and the touch versions but they didn't grab me the same way. Maybe just a different stage in life? I just try OpenTTD again though... Just to scratch that itch.