I had initially planned to use the tender from an existing locomotive, but after cutting it up I realize it would be more work than started from scratch.  

I made some steps from brass strips and bolted them in place.  The tender tank is made up from some tubing I bought from Plastruct - thicker tubes in the front and smaller ones at the rear.   Styrene sheet was then used to act as an inner wall between the tubes.  A larger sheet of styrene was used to make the top of the tender.  I added the switches that will be used for power and volume control.   A couple of tool boxes and the tender water valves were salvaged from the donor tender.

I wanted a working tender water hatch so I could use it to conceal the power and volume switches.  I started by laminating 3 pieces of .020" styrene around a couple of tubes I taped together to form a curved base. Next,  I cut the lid from some .040" styrene and attached it with some doll house hinges. 

I'm not a big fan of detail that you can't typically see, but the brake cylinder on the tender cried out to be modeled as it can be seen from the side.  Since I was going to model that, I decided I might as well add a few more visible pieces.

Simulated rivets were embossed on .020 styrene.  Needless to say, this is only a tad tedious.   The styrene was then wrapped around the tender inner form and glued in place. 

The top edges were trimmed with .060” half round styrene.  I built hold down cleats from styrene. I used some extra nuts and bolts from some Accucraft couplers for the anchors.  The tank spigot was made from some brass tube and a dress snap for the handle.   Finally, I used brass strip and rod for the tender ladder.   Grab rails were bent from music wire and glued in place.