Welcome, Cookiemuncher45 (interesting title that), to the world of Trainz.
Can I download custom routes/trains?
Yes. There are free routes, sessions (that would take a while to explain), locomotives, wagons, scenery of all types, that are available on the DLS (
Down
Load
Station) which you can access from the
Content Manager program as soon as you have installed TRS19 (or later). I just did a quick check and as of a moment ago there were 705,250 items you can access and 40,672 of those were rolling stock.
There are also Payware routes, sessions, rolling stock that are available through the
Content Store (or DLC for
Down
Load
Content) which you can access as soon as you have installed TRS19.
Do I need to purchase the DLCs to get more trains/routes?
No, if you are happy with the free offerings then you do not need to buy anything more. Some of the DLC routes and sessions come free as part of the TRS19 install.
How many free steam locomotive are there and what models are they?
30,757 locomotives covering a huge range of countries including UK, USA, Canada, Australia, Japan, most (if not all) of Europe, South Africa and probably a few other places (even Mars I believe). Steam, Diesel, Electric, horse drawn, hand pumped, etc. I am not sure what traction power the Mars locos use. You name it and someone has probably created it.
The best part is there is an entire community of Trainz enthusiasts that can offer advice, solutions, suggestions right here in these forums. There is an extensive Wiki devoted to Trainz and a Discord Server (which I don't use so I cannot point you to it).
You might want to take a look at the basics of using Trainz at the Wiki Page
Route and Session Basics or browse the whole Wiki for a variety of topics at
HowTo