Maintenance machines for railway track

a movable optical system for establishing a reference datum in relation to the surface of a roadway, comprising:

• a. a vehicle movable along a roadway,
• b. a support mounted on the vehicle and pivotable about a substantially horizontal axis extending transversely of the vehicle at a predetermined height,
• c. light emitting means mounted on said support and oriented to emit a beam of light along a first optical axis extending away from the pivotal axis of the support,
• d. light receiving means mounted on said support and spaced from said light emitting means and oriented to detect light along a predetermined second optical axis which converges with said first optical axis in a direction extending away from said pivotal axis of the support,
• e. an actuator mounted on the vehicle for adjusting the support about its pivotal axis whereby the said first and second optical axes can be caused to intersect at the surface of the roadway such that said receiving means receives along said second optical axis light from said beam reflected from the roadway, whereby an optical triangle is established, one corner of which is at the intersection if said first and second optical axes, and
• f. a device mounted on the vehicle spaced a constant horizontal distance from said horizontal axis and adapted to be adjusted vertically to maintain a predetermined vertical relationship with the plane of said optical triangle.