Figure 6.1  6.1.4 – Railroad-highway Crossings  6.1.5 – Prevent Trailer Skids  Railroad-highway crossings can also cause  problems, particularly when pulling trailers with low  underneath clearance.  These trailers can get stuck on raised crossings:  When the wheels of a trailer lock up, the trailer will  tend to swing around. This is more likely to happen  when the trailer is empty or lightly loaded. This  type of jackknife is often called a "trailer jackknife."  See Figure 6.2.  The procedure for stopping a trailer skid is:  Recognize the Skid.  The earliest and best way to  recognize that the trailer has started to skid is by  seeing it in your mirrors. Any time you apply the  brakes hard, check the mirrors to make sure the  trailer is staying where it should be. Once the  trailer swings out of your lane, it's very difficult to  prevent a jackknife.  * (From R.D. Ervin, R.L. Nisconger, C.C.  MacAdam, and P.S. Fancher, “Influence of size  and weigh variables on the stability and control  properties of heavy trucks, “University of Michigan  Transportation Research Institute, 1983).  If for any reason you get stuck on the tracks, get  out of the vehicle and away from the tracks. Check  signposts or signal housing at the crossing for  emergency notification information. Call 911 or  other emergency number. Give the location of the  crossing using all identifiable landmarks, especially  the DOT number, if posted.  Section 6 – Combination Vehicles  Page 6-2  • Low slung units (lowboy, car carrier, moving van,  possum-belly livestock trailer).  • Single-axle tractor pulling a long trailer with its  landing gear set to accommodate a tandem-axle  tractor.  2005 Model Commercial Driver’s License Manual