A/N This is TRS/model canon, with Thomas and Edward helping build the railway.
A new railway was about to be built on Sodor called the NorthWestern Railway. Engines had come to help build it, including Edward from the Furness Railway and Thomas, an LBSC tank engine. The latter's arrival was unintentional, but they had him do things like shunting cars of ballast and anything else he could do when work began.
Edward knew Thomas to be a much younger engine than himself, so he took on a mentor role, teaching him how to do things as needed. When he met the tank engine, that cheeky little grin on his face told him this engine was the age of a child. As they worked on building the railway, they became friends. Edward could see the smaller engine's good heart, even with his cheeky way. Thomas was a hard working engine, though not as big as Edward.
Due to being different types of engines, they had much different jobs in building the railway. Edward pulled the trains of ballast cars his friend Thomas shunted because the former was a tender engine and could handle things a tank engine like Thomas was unable to. The engines also had cars of the other materials, like rails. However, Edward was the kindest engine,so though they were different, they were inseparable in their friendship. The best times were when the two engines could work together or when the day was done and they could chat in the place where they slept. Time went by and more and more rail was laid. Other railway infrastructure was put up. Now, there it all was.
Thomas and Edward wondered what would happen now that their railway building was done. But soon, they knew. Thomas was considered lost by his original railway and sold to the new NorthWestern Railway for a nominal sum and Edward's old railway had no need of him, so they could stay put. This was quite unexpected news and brought floods of happy tears to their eyes. They were given NWR livery.
After being repainted, "I'm blue now, just like you!", Thomas said to his friend. Edward smiled at his little friend's joy they were painted in the same livery.
Other engines were added to the new railway and everything from then on is history.
A/N Due to the limited info I found, this is a one shot. I don't even know how many engines realistically are involved. I read on some fan comment somewhere about Thomas and Edward inseparable before Percy's arrival. Their chatting's like the engine form of pillow chat at a sleepover or perhaps between friends sharing an apartment more or less. Reviews are welcome, but not flames. Respect my Thomasverse and don't complain about my English. I'm not British, they are and I try to honor them being British, but I feel more comfortable sticking to my spelling and sometimes my words for things.