Welcome to the forums KJP1990
I'm like you, into RCT2 as well...I still play it a lot and I once designed the Titanic-Scenario for the game, does that sound familiar ?
If you know how to insert new things to RCT2 then you must know how that's done in locomotion. It goes the same way...
To use the new vehicles in the existing scenarios, you must open up the scenario in the scenario-editor (the globe on the far right in the main game menu), then point with your mouse on the save-disk icon and select "Load landscape". Next, point your mouse to the yellow folder icon with the arrow pointing up on the right and select a scenario from the list of excisting scenarios.
The landscape from the excisting scenario will now be opened.
Next, select the vehicles that you have downloaded (a maximum of 224 vehicles can be selected) and when your done with the selection of all the things that you want to choose go and save your scenario (one small advice, I suggest that you give the "new scenario" another name) and when you're done select your newly edited scenario from the globe on the far left in the main game menu and play the game.
You will now be able to play with your new vehicles.
By the way, here are a few links to other topics that you may find usefull, and they give your game a nice boost !"Here you can find most, if not all packs that are released for Locomotion"
A great list wich Illegal Alien, one of the people behind some of the greatest mods for locomotion has put together. Worth a visit !http://www.tt-forums.net/viewtopic.php?f=40&t=19614"How to add things to Road, Normal Gauge & Narrow Gauge?"
Another great topic by Illegal Alien for new players wich proved very usefull. Here's where I learned the basics for adding things to the game. Also worth a visit ! http://www.tt-forums.net/viewtopic.php?f=37&t=29348"Problem with Bridges"
If you have a problem with bridges or other things to add to the game you can take some time to check my topic on "Problem with bridges". A detailed topic which can be of some use for newbies or people who need help with adding stuff to their game:http://www.tt-forums.net/viewtopic.php?f=37&t=49943&hilit=problem+with+bridges"The Long Station Patch"
A brilliant patch for Locomotion created by Banjobeni and Crispy. Crispy gave me (Emperor Darth Sidious) permission to upload it here on the forums. With it you can create much, much longer stations in your game and this topic gives a lot of details of how to get it running:http://www.tt-forums.net/viewtopic.php?f=38&t=50627"The Valuables Mod V3.0"
The Valuables mod is my biggest (and most succesfull) mod so far. Information and download can be found in the topic. It's a mod that brings all kind of valuables to the game. There are a few members from the tt-forums who helped me a lot with this mod, after all it was a worth it. By a lot of members this was requested after the 2.0 version was removed from the forums. A truly must have mod for Locomotion:http://www.tt-forums.net/viewtopic.php?f=40&t=65622"The Food Mod"
This mod is an extensive mod for foodproduction and transport. It's still a WIP, but worth a look. This is not yet up for release but will probably released before the end of the year.http://www.tt-forums.net/viewtopic.php?f=40&t=66368
Have fun exploring, adding and playing the game !