Haven't played OTT(D-or-otherwise) for a while. May have to pick it up again.
The original TT (#1) was a particularly favoured time-waster of the day. (It helped that it was absolutely unbothered by what memory you had set up in DOS, EMS/XMS-wise, etc..., which saved time with re-editing the CONFIG.SYS et al.)
One simple way of playing, BTW, is to only pursue bidirectional transportations (mostly passengers, plus mail) at the beginning. Jump on absolutely every request to set up such a transport link between two towns (by your own favoured method, sometimes it's better with buses and sometimes with trains... but build airports when you can[1] for already large settlements with a significant intervening distance... I tend to neglect water-borne transportation for everything but service oil-rigs, but you might be able to get a good ferry service set up) and set that up as a to-and-fro service with fine-tuned "wait for load" settings on the vehicles involved.
Getting a "Circle Line"-like railway or an "M25"-like ringroad connecting a whole set of settlements might be a good thing to plan for, but initially just to-and-fro nets you the contract and the initial rewards that let you expand onwards and round and back again.
Of course, without service contracts for woods to lumber-yards you're not going to
get the villages to grow into towns, etc, but apart from contract-attraction requests that I think I can do well (especially if it looks like I can get in a secondary production network feeding off of it, while I'm there) I tended to scan the resource-source/sink map for woods that actually
were almost next door to the lumber yards, coal mines right next to the power stations, etc, and then set up a super-short shuttle-run between the two.
I tended to absolutely never mix train types (excepting mail-vans on the end of passenger trains), but for all I know I was missing a trick.
Don't know (can't remember, for the version I will have played) if this is possible in OTTD, but in original TT I found I could sabotage an opponents road-network by setting up a double-level-crossing over their route and looping an small, cheap, otherwise unused engine (and perhaps some carriages) over the road, thus:
║
┌╫┐
└╫┘
║
The engine speeds across the road again and again and again, and at
some point, any of the opponents' vehicles that use the road are going to be crashed through by it, if done right. Well, that's how I remembered it.
Anyway, off to download the latest OTTD now.
[1] Don't go borrowing too much in the early days, to do this, but bear it in mind)