Mileposts & Speed signs are associated to track conditions using the double arrow node. Both signs have variables accessed through the double left click on the object. Dependent on the scripting of the sign, these variables can be changed, but the speed sign gets its variable from the actual speed of the track, which is part of the assigned variables of the track, and is not able to be changed manually. The mileposts have a variable that can be changed showing a distance number, but is not offset from any one point, so actual distance offset from a source is not possible. A basic speed sign will assume the two speed variables related to the track when the double arrow node is placed on the track, the passenger speed and the freight speed, which is used by the AI trains and the user. These double arrows are present as soon as you place the sign, and both type signs need to have the node attached to the track with a left click. The reason for this is so the information the signs hold will be displayed on the track information portion of the F4 HUD. So, the placing of each sign is a left click to place the sign, and then a left click on the track to place the node. The speed sign is usually placed where track speeds change.
Also, remember, these signs may be different in each country you may be designing in.
These images may help show how things are placed:
Showing the placement of the milepost.

Showing the placement of the Speed Sign before the node is set on the track, zero numbers.

Showing the Speed Sign after the placement of the node, both speed variables show on sign. The colors bars on the track show the break point for the speed change you use the space bar set to three clicks to get the bars.

I hope this helps.