(“A way to salvage linear modules” is an older text on this exact same topic.)
I’ve often said that while the dowside to 5e being designed to be “all things to all people” is that it’s hard to find a group that plays in the style you want (with regards to things like fudging, narrative, improvisation etc vs a more strict challenge/exploration style game—rules light vs rules heavy is also an axis of contention), the upside is that there are a lot of monsters, maps, & modules you can use.
Here are some techniques I’ve found for rescuing modules written that don’t quite hit that clear&crisp “here is what is here” nature I’ve gotten so spoiled with from the best & blorbiest OSR modules.
Some modules write “there are two skeletons per adventurer”. Instead, I pretend that the book always believes there are four adventurers so in this case I would read it as “there are eight skeletons” even if five, three or only two people showed up to play that day. (Or select another number instead of four that you think fits your group better, the point is to select it before you start prepping these adventures and stick to it regardless of how many show up.)
Instead of foisting events on players “this happens, then this happens”, place events on either rolltables (encounter tables, random weekly event tables etc) or as things that can happen as specific places on maps are discovered. A linear “event sequence” module is useless on its own (for our playstyle’s purposes), but it can get torn apart and placed in a larger sandbox setting and really enhance that sandbox setting and cause some of the best and most memorable moments in it.
When there is stuff like “no matter if the players go east or west, place the keep in the direction they went”, that doesn’t really mesh well with our style so again, ripping the module apart and placing it in a larger setting is the key here. Place the keep in one of the directions and put something else from your blorb DM toolkit in the other direction.
Sometimes these books that I’ve previously dismissed as “unrunnably railroady” do come with sandboxy stuff, in an appendix, side-booklet, separate “world guide” or “city guide” book, intro section etc. By page count, the sandboxy stuff might be small and easily missable, overshadowed by the pages of pages of railroad BS, but as blorb DMs we might be used to, and even appreciative of, this brevity from OSR modules.
The trick is to flip the pancake and make this appendix, gazetteer or whatever it’s called the boss — if it’s insufficiently playable, then add in stuff from other modules, mash up encounter tables etc — and make all the “adventure” stuff serve it instead of the other way around. There’s no rush to “get the players through” those railroady chapters. The blorb playstyle operates on a “pull” model instead of a “push” model. The characters will find events by exploring maps through visiting places, or exploring tables through spending time in places.
And, sometimes a sequence of events as outlined in one of these books does happen to be perfectly cromulent to what happened in your game. Sometimes one thing does naturally lead to another, and your players do to zig when the text expects them to zig, and that’s just gravy! But with some of the above techniques you are more than happy for them to zag instead.
Some of these modules would be fine if it weren’t for one or two sentences while some of these modules require almost as much as grabbing a random novel or comic book off the bookshelf would. That’s why this trio of un-techniques is a toolkit instead of a procedure.
PS
Despite these techniques I don’t think a “rescued railroad” is a good
starter experience for new DMs. What makes them possible to even
work at all is having a good set of tools and familiarity with stuff
like creating encounter tables and such. My fave intro adventures are
B4 The Lost City and The Lost Mines of Phandelver, adventures that are
already location-based.