Disclosed is a system or kit for forming an assembly comprising a rail trolley engageable with a rail to be moveable therealong, the system or kit comprising: at least one portion which a body of the trolley is to comprise (body portion), the body portion(s) being configured to define at least one slot in said body; and one or more wheels connectable or connected to at least one said body portion, or connectable to said body, the system or kit including at least one component connectable to the body through at least one said slot in the body and/or in one or more said body portions, or connected to at least one said body portion through at least one said slot therein, such that the position of a resulting interconnection between the/each component and the body in the assembly is variable along the slot(s) through which that component is connected.