Can anyone recomend an engine spec?..

I'm creating a locomotive version of a Ffestiniog Railway slate wagon to use in gravity train simulations. Can anyone recommend an engine spec (preferably built in or on the DLS) that meets these requirements?
1) From a diesel or electric loco.
2) Has a top speed of 20-25mph.
3) Can move 250-300 tons on the level.
As this asset is only likely to be used in DCC mode or as an AI train 100% prototypical accuracy isn't required!

Thanks in advance
Ed

Hi Ed. I created a slate wagon loco for the gravity train with a lot of help from trz06003. it's on the dls - Ffestiniog (gravity loco) wooden slate wagon with animated driver,<kuid2:117948:100111:1>
It uses Salty 0-6-0 Dock Yard Shunter Spec,<kuid:142268:5101> by cmburges. It works great but the spec is Trainz version 2.2. Could use an upgrade.

You can see it run in route Porthmadog-Blaneau Ffestiniog - TS2010,<kuid2:117948:100125:3>
with session Ffestiniog 2011 - TS2010,<kuid2:117948:100126:2>

Both on the dls under username nawlins.

Cheers

I've been using your wagon as a basis for mine, including the script for the driver (is it OK to release my version with the script from your version?). However I've found from experience that the dockyard shunter spec can be a little underpowered when hauling longer trains (up to 100 wagons), particularly along the level, which although not prototypical is fairly necessary in trainz.