Sound deadening material under cork roadbed - Does it work?

A video showing a recent experiment I conducted to decide whether or not to add an additional layer of rubber under the cork roadbed in an attempt to reduce track noise.
Sourced a roll of 3mm (1/8") recycled rubber matting from Home Depot for $1.40/square foot. Set up a 12' test track with 6' of the track sitting on cork, on top of this rubber. The other 6' of track the traditional method of cork on plywood roadbed.
Results are that you can audibly tell the rubber reduces the track noise slightly and confirmed it with an iPhone app showing the sound was reduced by approximately 5 dB.
Track was Walthers Sinohara code 83 secured to the midwest cork roadbed with silicone. The cork was secured to the rubber and to the 5/8'' G1S fir plyood with the same silicone. Freight cars used for test were Intermountain, Exactrail, Intermoutain to try and keep the sound the same both directions of travel.
After posting this video, a lot of modellers left comments with other options or ideas for sound deadening. Here they are compiled into a list.
Home Depot recycled rubber matting:
3mm 24"x54'' = $13.00/9 sq.ft. = $1.40/sq.ft
3mm 27"x10'' = $31.97/22.5 sq.ft. = $1.42/sq.ft.
*5mm same sizes also available but more $.
Sonapan 1/2'' sound deadening 4x8 sheets
Homasote 1/2'' sound deadening 4x8 sheets
Auto Zone thin rubber matting for automobiles/dynamat/mass loaded vinyl. Mass loaded vinyl - $50.00/yard = $5.55/sq. ft.
Dollar store 1/2'' thick foam rubber interlocking pieces for childrens play areas
Fibre type sound deadening material
Put layer of rubber in between where benchwork attaches to walls to mechanically detach it.
Roxul sound batting underneath layout when complete.
Poured concrete subroadbed.
Heavy ceiling tile.
Blueskin waterproofing material from Home Depot:
6''x50'' roll = $34.00 ($16.35/sq.ft.)
9''x50'' roll = $46.41 ($14.85/sq.ft)
36''x66.7'' = $140.00 ($8.39/sq.ft)