A major challenge in the optical networking in the metropolitan area is the design of a robust and simple access scheme for building a flexible metro network. In this regard, we consider the bus-based approach of the Passive Optical Networks (PON) with the asynchronous CSMA/CA protocol. We present an enhancement of the CSMA/CA scheme, called LOCOMOTIVE, which takes benefit from the transparent nodes and unidirectional transmission of data signal on optical channel. Idea of the LOCOMOTIVE consists in creating trains of packets, where packets are glued one after another. As illustrated in this paper on simulation based results, the LOCOMOTIVE increases substantially the access protocol performance and achieves efficient utilization of network resources by minimizing the bandwidth fragmentation phenomenon.