GAME CRASHES WHEN STARTING
The game will usually crash for Windows 10 users. There are 8 solutions, so start with #1 and try each solution if it doesn't work, one by one. Windows 11 has not been tested yet.

#1 (FOR STEAM ONLY): open it normally for the first time,
Goto your Library, right click Redline on the left, goto Properties, goto Local Files, goto Browse Local Files, and open Redline.exe to start the game from there. You can launch it from Steam and keep the splash screen open, then do the mentioned steps to track your game time. For some reason, after doing this only 1 time, I was able to always launch from Steam afterwards.

#1 (FOR GOG ONLY): open it normally for the first time,
In GOG Galaxy right click Redline on the left, goto Manage Installation, goto Show Folder, and open Redline.exe to start the game from there. For some reason, launching from the GOG Galaxy client may not work the first time, but will afterwards.

#2: set the compatibility mode,
Do the same steps as above, but instead of opening Redline.exe, right click and goto Properties, goto Compatibility tab, check the Run this program in compatibility mode box, select Windows 98, check the Run this program as administrator box, and click OK.

#3: set the security permissions,
Do the same steps as above, but instead, right click anywhere in the Redline folder, goto Properties, goto Security tab, goto Edit, left click on your Windows login account name, check the Allow box next to Full control, click OK, click OK.

#4: having a wireless headset,
I figured out that Redline may crash if a wireless headset is plugged in. I have both a set of speakers and a wireless headset connected to my PC. Redline kept crashing upon starting up. After I removed the headset completely, Redline worked fine. I also tried to remove the speakers while keeping the headset, yet, Redline still crashed. So it must be the wireless technology that caused the crash. To remove an audio device, goto your Control Panel (Windows settings), click Devices (or View devices), then click (or right click) one of your audio devices (such as a headset) and click Remove. Make sure the device is physically disconnected from your PC too.

#5: check the video settings,
When opening Redline.exe, at the splash screen, click Display & Audio, and make sure Primary Display Driver is selected, and the correct resolution is selected.

#6: enable DirectPlay (a lot of older games check for it and it has been disabled since Vista),
Since Windows 10 disables DirectPlay by default, Redline may crash if it be detected (regardless if you plan on playing online or not). Goto your Control Panel (Windows settings), click on Programs and Features, click on Turn Windows Features On/Off on the left hand side, click the + next to Legacy Components, tick the box next to DirectPlay to enable it, click OK. Note that this also MUST be done to play any sort of multiplayer games.

#7: use a Voodoo wrapper (emulates an older graphics card),
Download the dgvoodoo2 zip at dege.freeweb.hu, goto the MS folder, goto the x86 folder, copy the DLL files (EXCEPT for d3d9.dll) and paste them into the same folder as the Redline exe. To set custom settings, extract the dgvoodoo.conf and dgvoodoocpl.exe files (main folder of the zip) and run the exe.

#8: use an IPX wrapper (emulates the very old IPX networking interface),
Redline may also crash if it doesn't detect any IPX protocols (regardless if you plan on playing online or not). Download ipxwrapper at solemnwarning.net/ipxwrapper, extract the files of the ZIP into its own folder, copy the 4 DLL files (dpwsockx, ipxwrapper, mswsock, wsock32), paste them into your main Redline folder, go back to the other folder and double click the directplay-win64 file (unless you're running a 32-bit Windows, which in that case use directplay-win32), click OK to add to the registry.

FORWARDING PORTS
Every router is different, but, the basic way to forward ports is to type in the URL that's physically printed on the back of your router (it'll be numbers such as http://192.100.1.0 for example), then use the username/password that's also printed on your router (default name is admin and default password is admin usually), then find a screen that says Port Forwarding (possibly under another screen such as Advanced or Access Control), then you should see a section to Add Rules. You have to type in the port # one at a time (for example, to forward port 2300, the External Port would be 2300 and the Internal Port would also be 2300). If it asks for the Internal Host number, that would be your computer's local IP (usually starts with 192, but, there should be a screen on that webpage that will tell you which device has which IP). You also need to select which Protocol the port number is (for example, 6073 would be UDP, or 47624 which would be the TCP/UDP combo). The 3 ports that will need to be forwarded for Redline are 2300 (TCP/UDP), 47624 (TCP/UDP), and 6073 (UDP).

DUAL MONITORS
Although it's possible that other issues are preventing Redline from starting, some people have said that using 2 monitors is the reason that Redline crashes. Personally, my monitor uses a DVI cord, but, I have to use a DisplayPort adapter in order to plug it into my PC, and Redline works fine (plus, I only have 1 monitor). If the above solutions don't fix your game, try only using 1 monitor, or if possible, don't use DisplayPort and try HDMI or DVI instead. I also saw that people with Nvidia cards had to disable G-Sync as well. These are unconfirmed problems/solutions though, and any input would be helpful.

TEXTURES LOADING SLOW
Sometimes, when a level is loading, the Textures bar will load very slow. To get around this, simply alt+tab in and out of the game (or ctrl+alt+delete), and it will automatically load up.

MONITOR FREEZING
Due to the overlay program that AMD has bundled with their graphics cards, if it tries to load when Redline is starting, your monitor may freeze. Disable the overlay before starting the game to prevent this, or, alt+tab from the program to restore the screen.

LAGGY MENU AND CONTROLS
For whatever reason, the menu for Settings is extremely slow, and so, remapping the controls (which is very necessary because the defaults are terrible) is a pain. If you click a key enough times while mashing the remapped button, it will change. An easier method is to open the Redline folder (steps mentioned above), and open the redline.cfg file in Notepad, and edit the keys. For on-foot, I suggest using WSAD for forward/back/sidestep left/sidestep right, Q/E for diving left/right, space to jump, mouse left click for fire, and 1-0 for changing weapons. For driving, I suggest using WSAD for moving, Z for left weapon, C for right weapon, X for back weapon, space for handbrake, Q/E for exiting, mouse left click for front weapon, mouse middle click for changing weapon, and mouse right click for auto-target. Also, be certain that Free Look is enabled under the Settings, as it will make aiming while driving much easier. If you have any problems, just download our default redline.cfg in the Downloads section.

MUSIC NOT PLAYING
The CD music will only play the tracks in order when a level is started. I suggest turning this off in Settings. During the game, scripted music plays at the start of the levels, and during crucial moments, and can't be shut off (since they're short anyway). That's a lot better than the same CD songs looping over and over.

SHOWDOWN LEVEL DOESN'T END
Even though patch 1.1 was supposed to fix the bug that the last level never ends, it will still sometimes happen. This is likely to occur if your turret is destroyed, you use the EMP on someone else, or you don't kill anyone. Try to avoid all 3 things.

AREA 51 LEVEL GETS STUCK
In the Area 51 level, there is a bug after fighting Golem and dropping into the underground, and the game freezes while in third-person view. If this happens, you need to update the game with the 1.1 patch.

SPECIAL WEAPONS
To use the zoom on the sniper rifle, just press the number 4 button to zoom in and out. To fly with the saw blade (in multiplayer and Freakway level only), click and hold the left click on mouse as if you're shooting, hold the W key to move forward, and hold the space bar to jump. In singleplayer, number 6 is the alien goo gun, but in multiplayer, it's a flash grenade that will blind other players.

MAC
Apparently, Redline got ported to the MAC, unofficially; however, the Steam version does not work with it. I don't own one so I can't test it, but, you can read more about it at portingkit.com/game/582