After reading a thread about the Hall class tender sounds, I decided to pop the top on my Polar Express tender to see if any improvement could be made to that.
As many will know, the standard Polar Express tender has a whistle that is extremely quiet at the best of times.
With the top off, I triggered the whistle, and placed a finger over each of the sound holes in turn. Besides altering the pitch, blocking one of the holes resulted in a noticeable increase in volume. I presume that this is due to more air being forced out of the remaining hole. I found that covering the hole on the right hand side (When looking from the front.) with a strip of electrical tape gave the most pleasant tone and greatest increase in volume. With this done, I replaced the top, refitted the 4 screws, and the job was done.
The beauty of such a change is that not only is it very simple, but it is quickly reversible without having caused any damage to the whistle or circuitry.
My Polar Express can now be heard when sounding its whistle. 
