oversize tank cars

I recently saw a picture of a PRR tank car from the late 60's riding on 6 wheel trucks. This car had a huge capacity but the caption said they were eventually banned by the FRA and now there are smaller capacity tank cars being used. Tne picture caption stated it was to demonstrate to shippers a larger size tank for for more capacity for the shipping dollar. Why did the FRA ban the use of the super size tank cars?

I believe there are still some massive (~45,000 gallon?) eight-axle tank cars in glycol service, though those cars are probably close to aging out. Was it actually an FRA mandate or just not economically viable?

In the early 1960s, the Union Tank Car Company (UTLX) introduced a series of "whale belly" tank cars which offered increased capacity over the standard cars of the day. Capable of carrying 33,000 gallons (125,000 l) (for example CSOX #31084) to as much as 63,000 gallons (238,500 l) in the case of GATX #96500, which had been conceived as a 'rolling experiment' of sorts. The largest tank car ever placed into regular service, UTLX #83699, was rated at 50,000 (189,200 l) gallons, and is now on display at the Galveston Railroad Museum -- first hit the rails in 1963 and remained in service for over twenty years. This behemoth is 89 feet (27 m) in length and weighs 175,000 lb. (79,400 kg) empty; the car, which rides on four two-axle trucks to distribute the additional weight, was used to transport such diverse substances as liquefied petroleum gas (LPG) and anhydrous ammonia.

http://www.rrdepot.com/wiki/Tank_car

GATX #96500 (largest ever built, 60,200 gallons)
http://www.rrpicturearchives.net/showPi ... ?id=436723
http://www.rrpicturearchives.net/showPi ... ?id=831428


UTLX #83699 (largest in regular service, 50,000 gallons)
http://www.1chan.net/rail/src/1291978266719.jpg

Scot

My guess here is simpler than my guesses about coal gons.

Three little letters: F--R--A.

The more you carry in one unit, the bigger the earth-shattering KABOOM or the spread of a toxic gas after one of these goes on the ground and gets opened up in the derailment.