Decided to modify my 5906 as outlined in previous posts by Joe Satnik and others. Here is what I did: 
1. Take 5906 apart (2 screws on bottom, usually covered up w/ a label).
2. Permanentally disable the weak, anemic and poorly constructed Normally Closed (N.C.) switch. I placed a plastic clip cover over the smallest part of the 2 part switch. Or you can simply bend the largest part of the switch out of the way. Either method will break the switches ability to make contact. The switch is now in a permanent Normally Open (N.O.) position---this is what you want. Throw away the plastic button and spring--you woan't need them any more.
3. Remove the thin red 22 ga. wire--replace it w/ a 2' (or what ever you think you'll need) 18 gauge stranded red wire. Do the same w/ the black wire, replacing it w/ 18 gauge black stranded wire. You need only novice soldering skills to do this. Close up the housing making sure the red and black wires are free and o/s the 5906.
4. I mounted this housing under my layout and ran the R & B wires thru a hole up into a Radio Shack "Project Box" (P# 270-1801) mounted about 3 inches from my ZW trans. I then installed a Ace Hdwr. 3 amp Normally Closed (N.C.) momentary SPST push button switch (P#3023728) between the two wires (bridging the wires). You must plan the location of this switch because you need enuf wire to run from beyond the switch to your trans and middle track wire. Then hook up red wire to trans + post and black wire to middle rail feed. Whalla--you're done.
Hold the button down for about a second and the bell starts making sounds. The loco does pause briefly until the bell starts, but is back up to original speed almost immediately. It stays on until you push the button again for a second or so. I modified both my 5906 units and placed both switches in the RS box. All 3 of my WbW and WbB diesel locos work and sound great w/ this modification---This is surely an improvement over the original 5906.
GTBob
1. Take 5906 apart (2 screws on bottom, usually covered up w/ a label).
2. Permanentally disable the weak, anemic and poorly constructed Normally Closed (N.C.) switch. I placed a plastic clip cover over the smallest part of the 2 part switch. Or you can simply bend the largest part of the switch out of the way. Either method will break the switches ability to make contact. The switch is now in a permanent Normally Open (N.O.) position---this is what you want. Throw away the plastic button and spring--you woan't need them any more.
3. Remove the thin red 22 ga. wire--replace it w/ a 2' (or what ever you think you'll need) 18 gauge stranded red wire. Do the same w/ the black wire, replacing it w/ 18 gauge black stranded wire. You need only novice soldering skills to do this. Close up the housing making sure the red and black wires are free and o/s the 5906.
4. I mounted this housing under my layout and ran the R & B wires thru a hole up into a Radio Shack "Project Box" (P# 270-1801) mounted about 3 inches from my ZW trans. I then installed a Ace Hdwr. 3 amp Normally Closed (N.C.) momentary SPST push button switch (P#3023728) between the two wires (bridging the wires). You must plan the location of this switch because you need enuf wire to run from beyond the switch to your trans and middle track wire. Then hook up red wire to trans + post and black wire to middle rail feed. Whalla--you're done.
Hold the button down for about a second and the bell starts making sounds. The loco does pause briefly until the bell starts, but is back up to original speed almost immediately. It stays on until you push the button again for a second or so. I modified both my 5906 units and placed both switches in the RS box. All 3 of my WbW and WbB diesel locos work and sound great w/ this modification---This is surely an improvement over the original 5906.
GTBob
Good luck w/ your loco.
