Quarter inch rubber matting for sound deadening?

 What track your using  and what your placing the rubber mat on would help. If placed over plywood. It's best not to secure the track with screws right into the plywood. At least long term. If the rubber compresses easily you may end up with less than level trackwork. Using screws to hold the track just in position. Not really tight or just adhering the track with glue and applying ballast and glue to secure it in it's final position. Then remove the screws when dry would be the way to go. I've always preferred homesote over plywood and roadbed over the homesote on the mainline. Homesote will hold brads or screws on it's own. I realize 30 years ago a lot of the products such as insulation foam and rubber matting weren't around back then. Nor the big box stores with more products to choose from. The only problem with this method. If you decide to change the plan. Removal of track or roadbed may be a pain.

KLINE Super Snap and regular tubular. Probably plywood cutouts for the base. Thanks for the reply. I think the Homesote is the way to go. Menards has  4x8 panels for $26.99.

If youre using tubular track you wont get the noise reduction you're looking because its hollow.

Well another change in my plans....

So track right on the wood I'm using? 

I can live with the noise.

Depending on how much track you have, you might consider some pre-made roadbed called Flexxbed. I posted a few questions about this material. For 1/4 inch thick it runs about a dollar a foot, but it is already cut to correct width with beveled edges.

Any roadbed will lessen the sound some, but as others stated, type track, how track is attached, ballasting, etc all contribute to noise level. Your room acoustics can make a difference such as a carpeted floor.