Linear Collider Alignment and Survey (LiCAS) R&D group is proposing a novel automated metrology instrument dedicated to align and monitor the mechanical stability of a future linear high energy e + e collider. LiCAS uses Laser Straightness Monitors (LSM) and Frequency Scanning Interferometry (FSI) [2, 3] for straightness and absolute distance measurements, respectively. This paper presents detailed simulations of a LiCAS system operating inside a Rapid Tunnel Reference Surveyor (RTRS train). With the proposed design it is feasible to achieve the required vertical accuracy of the order of O(200)µm over 600m tunnel sections meeting the specification for the TESLA collider [4]. 1 Principle of the LICAS-RTRS train operation In figure 1 the schematic view of the LiCAS train operating in the accelerator tunnel is presented. The train is composed of 6 cars, the distance between the centres of neighbouring cars is � 4.5m. Each car is equipped with 4 CCD cameras and two beam splitters (BS) constituting the straightness monitor. The straightness monitor measures the transverse translation (Tx,Ty) and transverse rotation (Rx,Ry) with respect to a z axis defined by the laser beam passing through all cars in a vacuum pipe. The laser beam is reflected back using the retro-reflector (RR) located in the last car, illuminating the upper CCD cameras of the straightness monitors. 6 FSI lines placed in the same vacuum pipe between each pair of cars are responsible for the distance measurement along the z axis (Tz). In addition a clinometer located on each car provides a measurement of rotation around the z axis (Rz). When the train stops in front of the wall markers it firstly measures the relative position and rotation of all cars with respect to the first car. This defines the local reference frame of the train in which the location of the wall mounted reference markers are measured next. This procedure is repeated for each train stop. Each marker is measured up to 6 times. Finally the coordinates of each marker, expressed in the local train frames are transformed to the frame of the first train (the global frame) by fitting them to each other under the constraint that wall markers have not moved during the entire measurement. 2 Opto-geometrical model of the LICAS-RTRS train In order to study the expected precision on the position reconstruction of the tunnel reference markers a simulation of the LiCAS survey train was performed. To describe the sensing parts of the train the Simulgeo [5] package was used which allows for modelling