An automated storage and retrieval system comprising: a rail system comprising a first set of parallel rails arranged in a horizontal plane and extending in a first direction, and a second set of parallel rails arranged in the horizontal plane and extending in a second direction which is orthogonal to the first direction, which first and second sets of rails (110, 111) form a grid pattern in the horizontal plane comprising a plurality of adjacent grid cells; at least one container handling vehicle (200,300) being configured to move on the rail system, wherein the at least one container handling vehicle (200,300) comprises a wheel arrangement (201,301) being configured to guide the at least one storage container vehicle (200,300) along the rail system in at least one of the first direction and the second direction; a service vehicle for movement on the rail system, said service vehicle being provided over the top of the rail system and comprising: - a container vehicle handling part for mechanical interacting with the at least one container handling vehicle (200,300) operating on the rail system, an operational part for controlling operations of the service vehicle, and propulsion means (6,7), characterized in that the propulsion means (6,7) allow movement of the service vehicle in any direction over the top of the rail system during operation, wherein the propulsion means (6,7) comprises: a first caterpillar track comprising a longitudinal extending endless belt, a second caterpillar track comprising a longitudinally extending endless belt directed parallel to the endless belt of the first caterpillar track, a belt motor driving the endless belts, wherein the first and second caterpillar track (6,7) have, when moving over the top of the rail system an overall length L exceeding the distance across two grid cells in the first direction and the second direction, wherein movement of the caterpillar tracks (6,7) over the top of the rail system signify that the caterpillar tracks (6,7) are supported on the rail system, but do not engage with the rails (110,11) themselves. a rail system comprising a first set of parallel rails arranged in a horizontal plane and extending in a first direction, and a second set of parallel rails arranged in the horizontal plane and extending in a second direction which is orthogonal to the first direction, which first and second sets of rails (110, 111) form a grid pattern in the horizontal plane comprising a plurality of adjacent grid cells; at least one container handling vehicle (200,300) being configured to move on the rail system, wherein the at least one container handling vehicle (200,300) comprises a wheel arrangement (201,301) being configured to guide the at least one storage container vehicle (200,300) along the rail system in at least one of the first direction and the second direction; a service vehicle for movement on the rail system, said service vehicle being provided over the top of the rail system and comprising: - a container vehicle handling part for mechanical interacting with the at least one container handling vehicle (200,300) operating on the rail system, - a container vehicle handling part for mechanical interacting with the at least one container handling vehicle (200,300) operating on the rail system, an operational part for controlling operations of the service vehicle, and propulsion means (6,7), characterized in that the propulsion means (6,7) allow movement of the service vehicle in any direction over the top of the rail system during operation, wherein the propulsion means (6,7) comprises: a first caterpillar track comprising a longitudinal extending endless belt, a second caterpillar track comprising a longitudinally extending endless belt directed parallel to the endless belt of the first caterpillar track, a belt motor driving the endless belts, wherein the first and second caterpillar track (6,7) have, when moving over the top of the rail system an overall length L exceeding the distance across two grid cells in the first direction and the second direction, wherein movement of the caterpillar tracks (6,7) over the top of the rail system signify that the caterpillar tracks (6,7) are supported on the rail system, but do not engage with the rails (110,11) themselves.