This is a Rectify Series drop-in speaker system for mid-production Athearn RTR SD40-2 and SD40/45T-2 Tunnel Motor locomotives. More than just a box, the enclosure features Diapasonic Tuning™, which allows the lower frequencies to accurately develop and resonate, clears the midrange of excess resonance and tunes the upper-mid/high response. The speaker uses the factory mounting screws and no modifications are needed to replace the factory speaker.
 
Note that the very early-run Athearn RTR SD40-2s have a narrower internal hood of around 17mmW with wide ribs, the speaker mount is further back and the speaker screws are spaced closer together at 10mm. Please choose "Early Version" if you have one of these. The later-run RTR SD40-2s have a smooth, wider internal hood just over 18mmW with the mount closer to the motor and a screw spacing of about 13mm. Choose "Mid Version" for these.
 
Your shell may have small "nubs" hanging from the roof where the speaker sits. These are left over from the molding process and may interfere with the shell seating fully down. Simply use need-nose pliers to reach in and twist them off.
 
The latest runs by Athearn using the dual-cube speaker/weight-size would use this speaker: https://www.scalesoundsystems.com/product-page/athearn-rtr-universal-sd39-40-2-current-run
 
You can add a front-mounted speaker by removing the small, screwed-on weight behind the cab.
 
The speaker comes fully assembled and tested.
 
VIDEO: https://youtu.be/sbGkyNWoL_Y