Surveyor Classic
In Surveyor Classic, the bulk tool is found in the Tools Menu.
Details using this tool can be found on the wiki here.

S20 Bulk Paint Under Spline Tool
S20 introduced a new bulk paint tool that will save hours of manual painting.

To paint ballast under a specific track, select the track spline, select the ballast texture to be painted, select your radius, select the area to apply the change to, click Begin! You can also apply any other texture under a spline such as grass along a roadside, or fence line.

Steps
* From the Surveyor Classic Tools Menu Select "Bulk Asset Update/Replace".
* In “Assets to update” list, use the Picker to select one or more existing splines, or drag more splines from the Track list.
* In “Asset to update with” select the ground texture you want to use as fill.
* Under the Paint Tab -> Radius, change the radius to the width you wish to paint. (The smallest radius will give a 10m wide texture).
* Click "Begin" to bulk paint the ground texture under the spline chosen.