The only free content is scenarios and routes available via the Steam Workshop or whatever can be found on UKTS or on reskin sites such as DPsimulation.
Yes, you do have to shop sensibly. Try only buy in the sales and read the descriptions carefully. The pricing tends to reflect the age and quality. This sim has been going for a long time now and some of the early releases aren't up to todays standards, hence why they are cheaper many of these DLCs are advertised as 'TS Classics.' This sim is a step-up from Trainz in terms of quality and realism, I'm led to believe.
In the current incarnation of the base game, you get: 3 modern well-detailed routes, one US, one German, one UK which is the North Wales Coast set in the modern period and comes with 3x modern DMU's.
Alternatively, there is this:
'Buy TS UK Classics Edition
Includes 4 items: Train Simulator, Train Simulator: Great Western Main Line Route Add-On, Train Simulator: Liverpool-Manchester Route Add-On, Train Simulator: Portsmouth Direct Line Route Add-On'
Neither of which are entirely your thing as there is no steam included, be also aware that the GWML and PDL are both quite dated routes in the sim.
In terms of the Steam Routes that I mentioned here is a brief assessment:
Western Lines of Scotland: Long, excellent route with mainline-running, secondary main line and branch line running. Very nicely detailed. Comes with a Black 5 and lots of rolling stock.
Riviera in the 50's: Medium sized well-detailed route. Includes mainline and branch line running. Comes with 5 different steam engines and lots of stock.
J94/Maerdy Route: Comes with advanced quality J94 and a short but interesting colliery route.
Weardale & Diesel Route: Comes with 200 miles of mainline and branch lines. Reasonably detailed. Mainly a diesel route but can be convincingly operated with steam. Comes with class 08, 25, 37, 101 plus rolling stock.
Netherfield: Nottingham Network Route. Reasonably detailed but modest selection of routes around Nottingham mainly focused on freight and local operations. Comes with 5 different classes of steam loco plus rolling stock.
There are a few others: Falmouth Branch, Woodhead, Corris branch, West Somerset Railway, Somerset & Dorset Railway. There is a lot more modern/diesel/electric DLC, but there's nothing to stop you running steam specials on those.
If you would like an honest review of any DLC that I own, I'm happy to help.