Jerry,
I vote for trucks or trailers. If you can find a late 1950s tractor trailer that'd be the ticket. Or else you could go to Toys R Us and bring your flatcar and check truck sizes in person.
Get you some pretty inexpensive 1:32 corvettes or mustangs:
http://www.aawesometoy.com/c-68-132-136-die-cast-cars-trucks.aspx
These firetrucks at the bottome are 1:24 scale, and a little pricey, but then again, it might be what you'd like:
http://www.idcow.com/trucks.html
Just a trailer:
http://bubbabigrig.com/cgi-bin/online/storepro.php
This place looks like some decent prices for trucks:
http://www.motormint.com/index.asp?PageAction=VIEWCATS&Category=208&Page=1
I like this set:
http://www.motormint.com/index.asp?PageAction=VIEWPROD&ProdID=1667
I vote for trucks or trailers. If you can find a late 1950s tractor trailer that'd be the ticket. Or else you could go to Toys R Us and bring your flatcar and check truck sizes in person.

Get you some pretty inexpensive 1:32 corvettes or mustangs:
http://www.aawesometoy.com/c-68-132-136-die-cast-cars-trucks.aspx
These firetrucks at the bottome are 1:24 scale, and a little pricey, but then again, it might be what you'd like:
http://www.idcow.com/trucks.html
Just a trailer:
http://bubbabigrig.com/cgi-bin/online/storepro.php
This place looks like some decent prices for trucks:
http://www.motormint.com/index.asp?PageAction=VIEWCATS&Category=208&Page=1
I like this set:
http://www.motormint.com/index.asp?PageAction=VIEWPROD&ProdID=1667