Graham.
Just have a look at the tender trucks.   If they DO NOT have wiper contacts on the tender wheels, they are the latest version and do not need new counterweights.

Thank you for the compliment.
It looks easy because it is easy.

The secret is to try and keep any wiring as neat as possible.
The only soldering necessary with the Thomas installation is the output of the ESC to the motor terminals.  Even that could be avoided if if the installer removed the track pick ups and fed the old track pick up wires back up to the top of the chassis.

I have recently converted a new Large Scale Thomas to battery R/C.

This thread at LSC shows how I did it:

http://www.largescalecentral.com/LSCForums/viewtopic.php?id=12432

What a great looking loco.  Nicely made.  Just runs way to fast at 12 volts.

Actually, unless I am mistaken, the AristoCraft socket was a co-design with Digitrax and then AC got their version back to front when they used them in their locos.
I believe the Bachmann design is faithful to the original which makes them correct and AC wrong. 
No matter how many thousands AC have made.
It may be a de-facto standard but that does not make them correct.

I certainly do not recommend a BASIC-3 for a three truck Shay.
Running light maybe, but not pulling a train.

So did you use  Phoenix P2/P2T?  Where did you get those parts?  Not listed by Phoenix.

Those are 6 volt Gel Cell batteries.
They do not last very long if not kept charged regularly.
Unfortunately you will need to replace the one you have.  I think you will find it very difficult to get a straight replacement battery.
If you do manage to find a replacement, as recommended by Sierra, make sure an external charge jack is fitted to the loco to ensure the battery is kept charged in future.

Rod.

The Shay equipped with a Tsunami DCC sound decoder should not be powered by an ESC that uses a PWM output.
That includes pretty well every R/C system made.

DCC decoders do not like PWM and can get "confused" such that they may cease to function on DC at all.
Also, you will have no control of any other functions at all.

JD.

You wrote:  ... Just completed an installation of RCS Basic 3, batteries, and Phoenix P2/P2T sound system.

The BASIC-3 is not really big enough for the 3 truck Shay. 

Did you mean:  ... Just completed an installation of RCS Basic 5, batteries, and Phoenix P5/P5T sound system.

Both the Bachmann motor outputs AND lighting circuits ARE wired back to front when compared to the AristoCraft sockets.
I believe Bachmann have it correct and all the AristoCraft sockets are back to front.
It actually doesn't matter very much as both motor and direction lighting can be set to suit.

The P5 is intended for use with either DCC or battery R/C.
It does not have any battery back up like the PB9.
What sort of on board control system do you have fitted to the Connie?

Here is how I did a Heisler for a customer.

http://www.mylargescale.com/Community/ForumArchives/tabid/100/Default.aspx?TOPIC_ID=44522

Me too!!!

Or a Class A Climax.

You can read all about it here:

http://www.mylargescale.com/Community/Forums/tabid/56/forumid/35/postid/100015/view/topic/Default.aspx

Really nice running loco.

This is the second version of the K-27 tender truck.

It is supplied on the second batch of K-27's that have the new counterweights fitted and a good way of identifying the locos with the new counterweights.

Thank you Mr Bach Man.

So. I take it your answer is confirmation that the output signals from the optical triggers are the other way around from the K-27?

That is not a problem for me.
The RCS/EVP PnP-3BK ESC's are set up for the signal being either way.