Sometimes objects stand in the way of the construction work or some old infrastructure should be demolished. There are several ways to remove these objects.
In every way the terrain gets reconstructed to the state before construction of the removed object as best as possible, if the removed object had modified the terrain.
Everything in the game world - except industries and the headquarters building - can be removed by bulldozing. Objects which can be bulldozed are highlighted in yellow when you hold the mouse cursor over it. If you like to demolish a construction like stations or depots, there will be a confirmation decision before it is actually removed. Press to confirm the removal or to decline removal. Pressing SHIFT while clicking on a construction will skip the confirmation requirement. To remove several assets without confirmation check directly after each other, you can hold down the left mouse button and drag the mouse.
When streets in towns are demolished, the adjacent town buildings and bus/truck stops as well as waypoints get removed as well. They each can be removed individually by clicking on them instead of the street. The same applies to tracks with signals and waypoints.
It is possible build over existing objects to get them removed if they collide. This works for anything that is not requiring a removal confirmation as explained above. When you upgrade streets in a way that they change the width and do not fit to town builings anymore, these town buildings are bulldozed automatically if necessary. The bulldozing cost is added to the building/upgrade costs.
Landscape assets like trees and rocks can be removed with the eraser tool. You can drag the tool along to clear a whole path of assets. There are several parameters available to adjust the size, form and sensibility of the eraser tool.