Is “lime” short for something?
In this case it is not a fruit Lime is a mixture of calcium hydroxide and calcium oxide produced from calcining limestone.
What is the meaning of “the floors of which had been covered with lime”?...Was the floor covered with this intentionally, or did this “lime” accumulate naturally?
Not really the place to ask, but the lime likely was from previously treating bodies. As people were left to die, lime would be put on their corpse to keep the smell of decaying flesh down. There is also some belief that lime can speed up decay and reduce bacteria growth, but this is disputed. Any way not all of the lime would land on the body or it would fall off when moved. This process had happened so many times prior that the floor was covered in it. lime tends to build into layers as it is chemically similar to cement and will harden over time the same.
What would be the consequences of this to the people in the railway cars?
Lime is very caustic and causes great respiratory distress when inhaled. It is used to capture the putrid scents of decaying flesh.
Today lime is still used at mass grave sites to capture the scent of decay and keep soil pH high. Low pH soil is an indicator for a mass grave as the decomposition products are acidic and lower the soil pH. Adding lime reduces this acidity masking (trying to mask) the presence of a mass grave.