Although there are rows of sleepers below, it is still too narrow because the train is very heavy.

The function of stones is to widen the subgrade, so that the stratum is not easy to sink and the railway is deformed.

2, there is another reason,

Small stones are harder than the road, and there are many holes in the middle. If they are made into cement foundation, steel and cement are likely to be damaged because of their different coefficients of thermal expansion and cold contraction.

Because pebbles can move relatively, they can cushion the deformation caused by thermal expansion and cold contraction.

In a word, firstly widen the subgrade, and secondly resist thermal expansion and cold contraction.

Another is to ensure the smooth operation of the train. Without ballast, the train will run very unsteadily and it is impossible to have a high speed. The other is to ensure that the subgrade will not sink.

The railway term of this kind of crushed stone you mentioned is "ballast", and its function and significance are: 1 bear the weight of train-rail-sleeper and transfer it to subgrade; 2. Drain rainwater into the catchment ditch beside the railway to prevent the railway from being soaked by rainwater; 3. Buffer the longitudinal impact of the train to make the train run more smoothly; 4. It is convenient for the maintenance and replacement of railway public works departments. Therefore, railways generally use stones as ballast under sleepers, and materials such as quartzite, granite and gneiss should be selected. In addition, the high-speed railway under construction has been replaced by concrete monolithic ballast bed. So as to evenly disperse the gravity from the rail to the ground, thereby ensuring the safe operation of the train.