Gravity Lines

Do they still have gravity lines in use? Portions incorporated in existing grades would be a good answer too.

To my knowledge there aren't any existing gravity lines in operation, at least not in eastern Pennsylvania.
The D&H Coal Co gravity line was rebuilt as a conventional railroad on mostly new grade but that branch has been abandoned.
Erie's Wyoming Division was built to replace the Pennsylvania Coal Company's gravity line and that is mostly abandoned also. There are places where the right of way can be driven and or hiked however.

did they operate on a roller coaster principle? short steep grade then a flat, easy grade going up to a summit a little lower than the start point, cresting that, they would drift down into the next bowl and so on til they got to the end of the gravity section. At least thats the way I understand it
wess

Generally speaking the loaded track and the return track, aka back track, would be on two different grades. In the case of the Pennsulvania Coal Company loads would be taken to the highest summit near Scranton via steam powered planes and then allowed to roll a considerable distance with brakemen controlling speed. Empties would be hauled up steeper planes and roll shorter distances between the planes.
The Mauch Chunk Switchback Railroad abandoned in the 1930's after surviving for some time as a tourist attraction, is easily traced in and around Jim Thorpe Pennsylvania. In their situation loaded cars rolled the entire distance by gravity with only the empties traveling over the two planes.
In some situations animal or locomotive mover cars on the levels sections.
There are a number of publications covering the Pennsylvania gravity roads.

Two great museums concerning the gravity roads can be found in Eastern Pennsylvania. One is the D&H CANAL Museum in Honesdale and the other in Jim Thorpe which has a model of the gravity roads in that area. Both have maps and literature,too. Neat stops if you are in the area!