It appears that there are two (at least) types of idler cars, and at least two names for idler/buffer cars? Type one, would be empty cars between the locomotive and somrthing potetially harmful, like hazmat,pipe, or rails. Type two, would be between extra heavy loads, to distribute weight on bridges.
How can super-heavy loads be run on lines not designed for them? An, in those case, why aren’t the idle cars run as empties?
For operational purposes, having several light cars ahead of a heavy car can cause problems. I saw an extreme example in Austin TX where a string of empty hoppers was trailed by some loaded rock gons. When the engineer accelerated after crossing the Colorado River bridge, the light weight hoppers stringlined between the engine and the rock gons and derailed.
Originally the space shuttle booster train alternated shuttle cars (on 8 axle heavy flats) with old box cars. The box cars had been ballasted with concrete to better balance handling. The trains are now configured with a buffer/gauge check car following the engine, then 4 loads, then another buffer, followed by 4 more loads, with another buffer bringing up the rear.
By the way, although I have not yet seen the accident report, it now appears that the weight of the locomotives - not the weight of the shuttle booster loads - precipitated the Myrtlesville bridge collapse.
dd
Gee, it’s going to be really embarrassing going to the marital aid store to find a suitable, umm, object to model this load. 
Hi guys,
sorry to keep you in suspense (long days at work). To answer your question about the loaded idler cars. The last transformer we just moved weighed 382,100 gross pounds. I apologize for the error, the loaded cars are not to weigh more than 263,000 pounds - still, ALOT of weight.
Going back to the original post - not all tank cars contain hazardous materials.
The real old-time “Trains” readers might remember when they ran a page-3 photo of an N&W freight train being operated by management during the 1978 BRAC strike with a tank car directly behing the engine. This was part of a lead-off article about the strike and its long-term effects. A union offical sent in a scathing letter which led off the following month’s “Railway Post Office” section, stating that FRA rules stating that hazmats can’t be hauled directly behind the power.
The next month’s “RPO” led off with a letter from John Fishwick, the president of the N&W, which identified the train, the signal location, listed the unit numbers, date, time past the signal, and the tank car number… as well as the fact that the tanker was an empty that had last hauled a non-hazardous latex compound used to manufacture water-based house paints. [:$]