Eliminating 9v battery in cars

Just put this unit into them.  eBay: 332358955185, $3.73 with free shipping.  It converts track voltage AC to DC.

AC/DC 3A Buck Converter Adjustable Step-Down Power Supply Module 24V to 12V

Will this converter work on conventional layouts? Or, since it is rated as "24V to 12V", can it only be used in command layouts? Also, would this be OK to replace the 9V batteries in the Lionel sound cars, being rated at 12V? (I know the original post asked about the QSI sound boxcars.)

Conventional operation this could be a crap shoot as you will frequently have less than around 10 VAC on the track for some locomotives.  You might look for the DC-DC buck/boost module and add an input rectifier and large filter cap, those will work with input voltages below the output voltages.

My YLB - RailSounds Battery Replacement was specifically designed to replace Lionel RailSounds batteries, and it will charge from around 6 VAC and anything higher on the track.  For Lionel stuff, you also have to be aware that the AC and DC ground are the same, that eliminates many of these cheap eBay solutions, but you can work around it.  The YLB works with any RailSounds board, both the older modular board set, and the newer ERR RailSounds Commander and the Legacy sound boards.

Not answering the OP's question, but for those that missed rtr12's "discovery" in a different thread, you can now get a 9V rechargeable battery with an integrated MicroUSB charging port.

I'm still not sure what to make of this!  Solution looking for a problem?  Or just what the doctor ordered?

Some 9V battery applications in rolling stock may not be suited for adding/converting to powered trucks.  The consumer gadget world seems to have settled on the Micro-USB charger for smartphones and such.  Hence, everyone already has the charger for this 9V battery. 

So depending on where the battery is situated/oriented in a boxcar, it could be fairly easy to access the charging port on the battery in situ.