For starters, let me say that I'm a veteran of the old Kuju/Railsimulators days. I've never been professionally associated with Kuju, RailSimulator or Dovetails games. I'm critical of them when I feel the need to be.
As a base driving game, TS2015 is a fair simulator. It does have it's problems and glitches but not as many as some are keen to point out.
The opening menu's are ok, pretty much self explanitory. One small problem I do see in the menu's is the confusing way of trying to select what to drive and where to drive and the lack of selectable filters attached.
The Academy(Tutorial) is a new addition to the sim. It's a good idea and a good starting place for beginners, although a little rough around the edges. This feature could include so much more and run smoother as far as transition between tutorials is concerned.
Once you head out into scenario driving, the sim is a class above. Much improvement has been made to the enviroment over the years, too much in the eyes of some who have struggled to keep up with sim's technology over the years, although lighting, shadows and weather can be excellent on the right PC. Driving is easy. Out of the box, the sim offers good variety and, since the inclusion of Steam Workshop, expands this variety even further. Career scenario and points scoring is a real bug bare and a negative in the sim for me. A simulator is exactly what it's supposed to be. You simulate the operation of the real world counterpart, you don't ask a driver to adhere to unrealistic timing to score points and head a leaderboard in real life.
A massive plus in my eyes is the Sandbox World Editor. The ability to create your own worlds whether it be your favourite or local route or something fictional that you have had in your head since you were a kid. The editor itself does have quite a steep learning curve, although, even the most inexperienced beginner can have a length of track laid and be running a Loco on it in no time. The learning curve is when you expand on this and create more and more complex layouts. I have found Sandbox modes in other popular simulators (not just trains) where the learning curve is very, very much steeper, especially for a beginner.
DLC seems to be a contencious issue with some and confusing for others outside of the sim. There is a great variety and to buy it all would require the cost of a decent second hand car. The point however, is you don't have to buy any of it to run the sim. I look at it just as I would a model railway. I get the basic oval track from Hornby in a box with the Flying Scotsman and a few coaches. I can run it to my hearts content. If I wish to buy some extra track and create a bigger route for my '4472', I can do. If I wish to have the variety of a Class 45 Diesel instead of an A3 Steam loco, I can buy one from the model shop, I don't have to buy the big brute of an American loco sat next to it on the model shop's shelf if I don't want to. You can expand as much and as little as you want. It's your choice.