OK!
Starting off, the main characters (which I'm sure you're already familiar with):
1. A male UP Big Boy steam engine who's the leader of the group, yet a gentle giant.
2. A female London and North Eastern Claude Hamilton 1880 steam engine who's the newest and youngest member on the railway and learns under the teachings of the UP Big Boy.
3. A male Metro North GE P32AC-DM 228, often handles the express.
4. A female SNCF TGV Sud-Est who works with high speed express, speaks with a French accent.
5. A male RENFE 1900 class diesel engine who works with freight loads, speaks with a Spanish accent.
6. A male R-46 Overhauled subway locomotive, he works in the city's rapid transit system and he's never seen with his "caboose car", even when hauling coaches.
7. A male Western and Atlantic "Texas" steam engine who is the oldest engine on the team, but still does his share of the work usually by the shunting in the yard due to his age.
Next, the story:
It starts off with the engines doing their usual work until they hear that their team is getting a new member who will be a shunter in the yards. The new engine (the Claude Hamilton 1880) arrives and is immediately put under the teachings of the Big Boy. Though the new engine is intimidated by the Big Boy's size at first, the two quickly get along and the new engine is sent to the repair shed to get fitted with a "universal coupler" so she could attach to the rolling stock. While she's getting fitted with this, the new engine asks the Big Boy why he doesn't have a tender. He responds by saying that he and the oldest engine were fitted with powerful solar electric batteries that would allow them work without the need of tenders, though only at short distances.
After getting her new couples, the new engine gets her usual coal-burning firebox replaced with a new one designed for burning vegetable oil (for environmental reasons) and is given the same solar electric batteries as the other tender engines. Despite how weird it felt to her, the new engine quickly gets used to her new design and quickly gets to work. While she's practicing her shunting, she meets the rest of the team, mostly through her mistakes. She accidentally bumps her rolling stock into the Spanish-accented 1900 class, who shrugs it off, having handled freight trains for a while, she the subway engine by shunting his coaches a bit to hard into him, who also shrugs it off, she meets the TGV by running out of fuel on her line, but she doesn't take is well since she's a high speed engine. The new engine also meets the oldest engine and Metro North engine, which are not through accidents.
After a while, the new engine is given the chance to be a member of the team by proving how well she can work. The new engine quickly does her best to prove how well she works by shunting rolling stock and retrieving coaches (The TGV still disrespects her though). But she soon gets the chance to pull her own (temporary) train: A trio of coaches. The new engine proves to do well at this, but her chances with the rolling stock causes trouble for her since the heavy cars prove too much for her on a hill. In the engine sheds that night, she's still upset about her mistake with the rolling stock. But the other engines cheer her up with the 1900 class admitting he had the same trouble with freight cars at first.
The other engines also admit other things they've had trouble with, such as the Metro North engine's fear of rain due to an accident he had and the TGV's bad attitude is because she's afraid of being late. The next morning, there's a rain storm and the engines are advised to be careful on the rails, after setting out, the new engine notices that the Metro North engine didn't come out of the shed due to his fear of rain. The new engine helps the diesel engine get over the fear while holding onto his coupler the way out of the shed, the engine gets over his fear and sets to work quickly. The next morning, everyone goes about their duties until the new engine notices that the TGV was having trouble with her brakes after she had retrieved her coaches. Knowing that she was in trouble, the new engine ditches the rolling stock she's shunting and steams after the TGV as quickly as she can.
The new engine tries to slow the TGV down by connected to her rear coach, but the TGV crashes before she does so (thankfully the coaches were empty). Knowing that she couldn't do this alone, the new engine quickly heads back to the yard to get help. The other available engines join in and help her get the TGV back onto the rails and the new engine hauls the engine to repair shed. During the travel, the TGV asks the new engine why she's helping her even though she had disrespected her, the new engine replies by saying that the Big Boy taught her that even though and engine may have a bad attitude, it's best to help them out either way. After that, the TGV starts treating the new engine with much more respect, even being patient while she's getting her coaches. After that, the new engine is finally allowed to join in on the team and is given a new paint job as a result to show she's a member of the team (even though it's not possible to recolor the engines in Trainz 2012 by my knowledge), even getting the chance to travel along the lines primarily as a passenger train by using the same three coaches from before.