So we all know that in 2003 plarail went and upgraded their engines 1-6, but there are a few more variants than one might expect.
In fact the first instance of these upgrades occurred in 2001 with the angry Thomas and powerful Gordon, so we'll include these too.

1.THOMAS

The last variant of Thomas that came before the upgrades featured a moulded face and stickers that included back lining

All these features were carried in 2001 on the angry Thomas witch is the transitional variant of the plarail Thomas models. Additionally it also features the original tomy logo on the underframe. Unfortunately, like all transitional variant models it fades quite easily!

After about 2003 we would get our first Thomas with printed side details. This would also mark the introduction of the new logo and the removal of the backlining, these would soon all be applied to the angry Thomas. I also believe that the grayish white they used would also be lightened.

Around 2012 Thomas would get a upgrade or at least in my opinion a downgrade to a new set of trianghiular eyebrows and smaller pupils and the original darer gray would make a comeback

I would also like to add that I have this surprised Thomas W/ eyes facing down from 2014, that has a lighter color and a white footplate

2.EDWARD

pre 2003 release:

Edward's transitional variant features a white drawbar and insulating sleeve, and like the pre 2003 model smaller eyebrows

*Note model shown is faded

After that the model gained larger eyebrows and the standard black drawbar witch it carried up until it was discontinued in 2018

3.HENRY

model before upgrades

Transitional model (white drawbar)

Final model

4.GORDON

Unupgraded version:

2001 powerful Gordon. The model features a faster and more powerful motor an exclusive reinforced coupling added weights and a fantastic face. It also had the old tomy logo, stickers for the wheel arch lining and an uninsulated white drawbar

Transitional variant: stickers for the wheel arches and a white insulated drawbar. It's definitely hard to find one that's not badly faded

2004-2012 variant: painted wheel arches and a black drawbar

pos 2012 variant: features a slightly different face witch looks more like his S2 laughing face

5.JAMES

original model

*Note model shown is a Trackmater(Tomy) release

Transitional model (white drawbar)

pos 2003-2012 (black drawbar)

The frankly horrific pos 2012 model with an ~~upgraded~~ face and slightly different tender printing

6.PERCY

original tomy (moulded face)

*Note model shown is from my scrap box

Transitional model (moulded face)

*Image from: https://www.blueplastictracks.org/thread-4564.html?highlight=plarail+percy

pos 2003-2017 release

I'm also going to add that the first batches of the 2nd generation talking engines also featured the transitional models of Edward, Henry, Gordon and James