Crossovers began in the Community in early 2014, and were originally popularized by MrMPS2002, Oliver Duck, and jlouvier. In them, two users make a Collaboration review on an item together in a professional, well-edited manner. Crossovers started as an over-arching storyline, referred to as The Crossover Saga. Today, crossovers aren't as popular as they once were, however many people still release at least one crossover a year.
The Crossover Saga
Knapford Express Coach Review
This was the first official crossover in the TWRC, and it was between MrMPS2002 and Jlouvier. They reviewed the Knapford Express Coach. This review proved to be wildly successful, and opened the door for new crossovers to come.
TYR: Big City Engine
This crossover was between MPS and Oliver Duck and was special in that it was built up in several videos prior. MPS appeared to "bug" Ted in several of his reviews before the crossover, and he finally "accepted", and they reviewed the Big City Engine together. This review is well known for the "Do a Crossover With Me!" joke.
TYR: Sawmill with Dumping Depot
This crossover was between Jlouvier and Ted, and they reviewed the Sawmill with Dumping Depot. There were some people talking about the fact that Wooden Railway Reviews had already reviewed the Sawmill shortly before the review came out, but Ted and Josh talked about it in the review. The end of this review built up to another crossover between MPS and Ted. This review is known for the joke where Josh hires the Troublesome Brakevan Brothers to put Billy in the Sawmill.
D199 Review
This crossover is the biggest one so far, and featured a total of six people involved, as reviewers or otherwise. MPS and Ted reviewed D199, and EricPierre53, WRR, Josh, and WoodenRS cameoed. This crossover is the best-received one so far, and is well known for many jokes, including the "Make it Rain" and the "Mexico Wooden Railway" jokes.
Knapford Express Coach Review! (Feat. Oliver Duck)
After the D199 crossover, Eric wanted to do a crossover with everybody who had done a crossover so far, and his first "victim" was Ted. They decided to review the Knapford Express Coach, even though MPS and Josh had already reviewed it, since Eric and Ted had never reviewed it. This crossover is known for the "Don't be an 808, kids," joke.
Roundhouse Review
This is part two of Eric's "crossover trilogy", and this time he does a crossover with MPS on all three editions of the Roundhouse. However, halfway through the review, evil versions of MPS and Eric called "Nega-Matt" and "Nega-Eric" replace them and do the review negatively, with the exact opposite opinions of the real Matt and Eric. This crossover is known for Nega-Eric and Nega-Matt's appearances.
Oliver Review
This crossover is the first crossover to ever feature Percyno6. Along with Ted, he reviews Oliver. Towards the end of the video, MPS makes an appearance and shoots Ted. Ted ends up surviving, but receives a new set of clothes. This crossover is known as the first one to have Percyno6, as well as for Ted's regeneration at the end.
Hector Review
This crossover features MrMPS2002 and Percyno6, with Ted making a guest appearance for the backstory. After MPS locks Percyno6 in a room, the two do a crossover. Once the crossover is finished, MPS leaves Percyno6 trapped. Dr. MPS eventually frees Percyno6 from his locked room. This crossover is known for having Percyno6 reference House of Cards by talking to himself as well as Ted's over-reaction to the backstories.
Logan and the Big Blue Engines Set Review
This crossover is the first official two-part crossover. After Eric's TARDIS malfunctions and drops him off in the middle of nowhere, he meets up with MPS, who is also there looking for Rex. The two decide to review the Logan and the Big Blue Engines set while Eric works on his TARDIS. This crossover is known for having MPS and Eric meet up in real life for the first time, as well as for having MPS sing "Happy".
Henry the Green Engine Review
MrConductorFan1406 is taught in the ways of reviewing by WoodenRailwayReviews in order to get revenge on Eric for stealing his costume. After a training montage, Vinnie and Rob do a crossover on Henry, before Vinnie sets off on his quest to defeat Eric. This crossover is known for Vinnie traveling across America as well as for Rob dressing up as a sensei to train Vinnie.
Transfer Table Review
MPS and Eric are trapped in Eric's room and forced to review the Transfer Table by MPS's evil mad scientist doppelgänger, Dr. MPS. Eric and MPS manage to defeat him and return to their respective homes. This crossover is known for being part of MPS and Eric's meet up, as well as for MPS and Eric's fight against Dr. MPS.
BoCo Review
MultiGreenThunder is still looking for someone to do a crossover with. After being denied again by MPS, Multi turns to Percyno6 to do a review on BoCo. Once the review is over, Eric appears and requests a crossover with Percyno6. Percyno6 doesn't answer, instead the words "To be continued" finish off the crossover. This crossover is known for being MultiGreenThunder's first crossover, as well as the one in which Multi's plans of taking over the world are revealed.
Ryan Review
Continuing where the BoCo Review left off, Percyno6 has been trapped by Eric. When Multi finally leaves, Eric said he actually never wanted to do a crossover and only saved Percyno6 from the guy with a creepy mask. At first they agree not to do a crossover, but they end up agreeing to do a crossover in the end. Once the crossover is over, Eric brings up how both of them have the same Beatles shirt. Percyno6 decides to switch to a different Beatles shirt, the same one as Vinnie. The shirt reminds Eric of his crossover with Vinnie. After Percyno6 says goodbye, the video cuts out mid-sentence as Eric is saying Vinnie. This crossover is known for Eric mistaking Ryan for Thomas and Percyno6 changing shirts.
Introducing James Review
Following off of MrMPS2002' solo review on Colin the Crane, MultiGreenThunder steals MPS' last can of Mountain Dew Baja Blast in an attempt to finally get MPS to a crossover with him. MPS agrees to the crossover in return of Multi giving him his precious Baja Blast back. The two end up reviewing Introducing James, but once the review is finished, they're not sure how to end the video since no users show up to foreshadow and set up a future crossover. Multi suggested that he and MPS do another one, but MPS yells about how he's sick of people annoyingly begging him for crossovers. Ironically, TurtlesandThomas shows up to point out how MPS often annoys him for crossovers as well. MPS says he gets it and storms off angered. A post credits scene shows that Multi also stole MPS' weapons upgrade for Jarvis. This crossover is known for Multi and MPS singing AC/DC songs.
Stanley Review
Multi is planning on doing another crossover to further his evil plans when he is stopped by an unlikely hero; Annie C, who plans on "bringing him to justice" in response to all his previous crimes (i.e., stealing MPS' weapons upgrade for JARVIS.) The two banter and eventually end up reviewing Stanley. Afterwards, Annie again states her intentions to deliver Multi to the legal authorities, but before doing so, Multi reveals a shocking secret, and then transports Annie by means of a purple beam to a nondescript basement. The crossover ends with Multi ordering a pizza for an apparently random reason, and we are left with no clue as to whether Annie eventually got out of the basement or not. This review is known for its many uses of the "Welcome Stanley" joke.
Non-Crossover Saga
Thomas 70th Anniversary Gift Pack Review
MPS and Turtlesandthomas team up to review the Thomas 70th Anniversary Gift Pack, with turtles camping out in a tent. However, turtles keep hearing noises outside his tent, and halfway through the review, goes out to investigate. turtles finds a ghoul in the woods, which mauls him. MPS simply finishes the review like nothing happened. This crossover is known for referencing the Blair Witch Project with turtles's scene.
Works Shed Review
Ted and Woodenrailway123 meet up in Britain to review the Works Shed. This crossover is known for being the first crossover review to feature two users meeting up, and is also known for Ted trying to give the backstory of the Works Shed before Tom interrupts him.
Sodor Bay Tugboat Review
MPS and WoodenRailwayReviews team up to review the Sodor Bay Tugboat while Vinnie is training. Eventually, the review dissolves into Rob ranting about the tugboat not floating. Ted also makes a cameo to try and talk about the Tugboat's backstory before he is dismissed by Rob. This crossover is well-known for Rob and MPS's homage to Jaws in the beginning of the review, as well as for MPS singing "How Could This Happen to Me" at the end.
2004 Rheneas Review
Set after the events of Thomas 70th Anniversary Gift Pack Review, MrMPS finds Turtles on the ground in a woods. He doesn't remember what happened between him and the ghoul earlier. The two return to their houses and MPS, once again, wants to do a crossover with Turtles. Turtles doesn't want to, but agrees to it in order to get MPS to shut up. The two review the 2004 Rheneas, but throughout the review MPS keeps hearing strange voices. It also appears that something is slowly attacking the residents of his house as well as J.A.R.V.I.S. After the review, Turtles leaves and MPS goes to investigate what's going on in his house. He is soon attacked by a mysterious entity. The scene cuts to Turtles, who is wondering if he should helped MPS back there, he decides not to listen to his "pesky" conscience get the better of him.
Stafford Review
Woodenrailwayonly and Percyno6 review the Wooden Railway Stafford, all while trying to avoid saying the term "crossover" and alerting MPS to their crossover. They fail, and MPS shows up at the end to scold Percyno6. This crossover is known for being the first video where WRO appears on camera, albeit behind a mask.
D261 Review
This was a crossover between Crimson2091 and DuncanWoodenRailway, the start of DuncanWoodenRailway's crossover arch after his previously failed one. This review was uploaded on his birthday and ties into that fact. The crossover was hinted at in July 2015, and the crossover touches on that in it's opening skit. John then receives a message from an anonymous person to meet at a certain place at a certain time, which ties into his James Meetup Review.
Donald and Douglas Review
Done with the desire to make TWR reviews, Turtlesandthomas decides to finally leave his basement and go out into the world. However he is stopped by Annie C., who wants do a crossover with him. Turtles is against this idea, but eventually agrees. The two end up reviewing Donald and Douglas. At the end of the review, Turtles still wants to leave and do a presidential campaign, but he ends up deciding against it. However, much to his dismay, turtles still becomes president at the very end of the review. This review is known for Turtles becoming president and the first crossover to feature Annie C., who wears a fox mask throughout the review.