While all Whittle Shortline Trains are compatible, the Series 44 cars are larger and more scaled to our C-44 Engines.  Pretty cool if we do say so ourselves ;)
TTX Series 44 Boxcar

TTX Series 44 Boxcar

$24.95

Canadian National CN Rail Series 44 Tanker Car - 5"

Canadian National CN Rail Series 44 Tanker Car - 5"

$24.95

ICG Series 44 Boxcar

ICG Series 44 Boxcar

$24.95

Series 44 GATX Tanker Car - 5"

Series 44 GATX Tanker Car - 5"

$24.95

Procor Series 44 Tanker Car - 5"

Procor Series 44 Tanker Car - 5"

$24.95

Southern Railway Series 44 Boxcar

Southern Railway Series 44 Boxcar

$24.95

ADM Series 44 5 Inch Tanker Car Wooden Toy Train by Whittle Shortline Railroad

ADM Series 44 Tanker Car - 5"

$24.95

Gulf Series 44 Tanker Car - 5"

Gulf Series 44 Tanker Car - 5"

$24.95

Norfolk Southern Series 44 Tanker Wooden Toy Train by Whittle Shortline Railroad

Norfolk Southern Series 44 Tanker Car - 5"

$24.95