When this happens, no brakes will work. The station brakes don't work, so just put some brakes in front of it. Bad luck, they also won't work. No brakes will work on this coaster and you can't simply close/open the coaster to reset the trains to the station. If the difference in speed of two colliding trains is more than 30MPH (confirmed in decompiled RCT2 source code), one of the trains will explode. There are two ways to fix this:

• Only use one train on your coaster (not recommended, decreases capacity)
  
• Don't use any brakes on your rollercoaster and make sure that when testing the coaster, the train enters the station with less than 40 km/h (20mph). If it enters the station too fast, either use shorter trains (they lose speed faster), extend your coaster or lower the chain lift. Adding brakes doesn't help. (recommended, use max trains possible)

Here's an example of a safe coaster:
Rollercoasters like the shuttle loop won't crash due a brake failure, though I still need to confirm that myself.

When using multiple trains on a single rollercoaster, I strongly recommend only using a chainlift directly after the station. If you use a chainlift in the middle or at the end of the track, multiple trains might block each other and the last one might fall back, either causing a crash when the next train comes along (can already happen when the last train waiting on the lifthill is also the first) or blocking all your trains at the middle of the track.

Fix:

• Run with only one train
  
• Increase the minimum waiting time so the the next lifthill is always free when the train departs from the station or comes off the previous lifthill.
  
• Brake-lift combination, see below.

You can use chainlifts at the end or in the middle of the track by adding some brakes (use 28km/h) and flat chainlift tracks in front of the chainlift. At least as long as the maximum lenght of trains that may need to wait there. Let me explain this:

• Brakes and chainlifts cannot fail at the same time.
  
• When the breaks fail the chainlift will keep the trains moving.
  
• In the case of a safety cut-out the brakes will prevent the trains from crashing, the flat chainlift track will make the trains continue after the safety cut-out is over.

Here you see a single brake segment on 28km/h, a few flat chanlift segments and then the chainlift to the station:


If you use this you can even use brakes in your coaster, as long as you make sure it doesn't enter the station with more than 40km/h if the brakes fail. Why? If the brakes work normally, it wouldn't reach the station without the chainlift. It 'gains' some extra speed from the chainlift making it able to reach the station. If the brakes fail, your coaster is so fast that is doesn't go slow enough to get some extra speed from the chainlift. It would only be a problem if the brakes and chainlifts fail at the same time, which isn't possible.

Image coming soon!

Can a coaster crash because of this? Yes (but it's rare).
The train second train will either crash when it hits the other train (>50km/h) or come to a complete stop (or in the worst case, fall back and make the next train crash). There are multiple ways to prevent this:

• Only run with one train
  
• Make sure the track goes down before entering the station so the trains will pick up speed again, but make sure that the trains enter the station with less than 50 km/h.
  
• Add some lifthill segments, either flat or on the last hill.

In other words, make sure the trains can pickup speed again if they stop right before entering the stations, either with gravity or a chainlift.

Rare, but it can happen: crashes caused by the peeps riding the coaster!

• Reverse incline shuttle coasters:
  
  When all peeps are at the back of the coaster, the front cart might fall off at the end of the track. Just make sure the track is high enough to prevent this.
  
  
  
• Continuous circuit:
  
  It only happened to me once: a train that simply doesn't have enough speed to complete the track. The coaster works fine with empty trains and full load, but with half load (I don't know if the peeps were in the front or the back) it doesn't always work. (screenshot coming soon)
  You can either run the coaster with one train (you get a message if trains don't return to the station), make sure the coaster goes over every hill with at least 20km/h or check 'wait for full load' and uncheck 'maximum waiting time' to make sure every train leaves with full load.

Here are some pre-built rides that need some modifications to run flawlessly. It will take a long time before they're all here but at least here are some examples other than just running with one train:

Dynamite Dunes:
Dynamite Blaster can crash on a safety cut-out. Increase the minimum wait time to make sure the train is halfway the lifthill when the next one leaves.

Diamond Heights:
Raised the station

Bumbly Beach:
Raised the station

White Water Park / Aqua Park:
Added a few flat lifthill segments to prevent the boats from locking up there.

Karts & Coasters:
Fewer cars per train, Lumber Jack did need some more editing, so run with 1 train until you can afford it.

Funtopia:
3 cars per train (note that the train is still going up in the image, it enters the station with a speed lower than 50km/h). The lifthill had to be extended (not visible in the image).

Hounted Harbor:
3 cars per train, no other edits were made for this coaster

Most of the pre-designed coasters are designed to be compact, but not to be safe. Here are some coasters that will crash or lock up if you run them for some time (incomplete list, and I won't do u(xxxx) coasters):
Steel mini:
Crazy Caterpillar

(keep in mind that most will crash)
To be continued! (if you want to contribute, just comment which coasters crash at some point if you don't edit them)

(I would recommend to always build a custom coaster, unless you saved a great design that is made to be safe, cheap and to fit in multiple parks, something like the blue steel mini example in the brake-lift-combination section)

Almost every rollercoaster I post is safe. Here are some more examples with the reasons why they are safe:

Only 1 train:
In the second image the station brakes failed (mouse is hovering over the rollercoaster), you don't want that to happen with a train waiting in the station...

This guide is initially meant for those who want to do everything to prevent crashes. There are crashes listed here that only happened to me once in the 100+ hours I've played this game. For the casual game I'd recommend only reading the sections about station brake failures and safety cut-outs. If you don't know anything about these 2 they'll cause about 95% of the crashes you'll experience. Only reading the section about station brake failures will probably save you ~60% of the crashes. And even if a coaster crashes the game isn't lost yet, however there is a chance that it'll bring the park rating below 600 on the last day. Often clicking on the messages that appear at the bottom of the screen can help you to sell the entire coaster right before the trains collide, which is what I did many years ago. If you think it's fun to design the safest coaster, read the entire guide, but most importantly:

Don't forget to have fun while playing RCT!

Thanks for reading!