These days, it's quite easy and VERY sensible to design a web site to scale for mobile, tablet, desktop and even people who have very large screens (many people now have 2K or 4K monitors).
As long as you design this from the outset, you'll have a site that doesn't need lots of testing for different devices - including devices you've not yet realised will be a 'thing' - or have features that only work in one mode and not another.
Given the smallest screen I have is now a 720x1280 pixel one, and the highest resolution is 1440x2560 (which is higher than the PC monitor I'm now using to write this!) there's less need for a dumbed down mobile page unless it's all about speed or for accessibility reasons.
And most apps are now just a front-end to take you to a mobile site. This is fine until the site goes down during disruption, where it then makes more sense to have a standalone app that gets data in the most simple, compressed, format possible.
I don't think any TOC has yet got this right, and that especially goes for the online booking engines for tickets. Even with a near 100 megabit Internet connection, you won't see these working quickly!