11  EP 3 800 002 A1 12  substantially directed in a main beam direction B of the laser beam 8 (i.e. optical axis) at the passage 6, prefer- ably at a main flow velocity of at least 100 m/s, more preferably at least 200 m/s, for example about 250 m/s. [0077] As is shown in Fig. 2, also, in an embodiment, during operation, a fluid flow of fluid flowing through the beam passage 6 is at least partially, and preferably con- trollably, directed towards a by-product target area so thatbeamdeliveryby-productsare atleastpartiallyurged towards the by-producttargetareaby the fluid flow. ,The fluid flow e.g. encloses an angle  θ  with the optical axis of the laser beam 8, for example an angle  θ  in the range of 1-30 degrees. [0078] A direction from the beam passage to the by- product target area may substantially correspond to a movement direction M of an object to be treated wit h respect to the system 2, in case of such a movement. [0079] It is preferred that the external laser beam pas- sage 6 includes or provides a constriction 24 in a fluid flow path through the beam passage 6. As is shown in Figure 4, to that aim in particular the beam passage 6 includes and/or is part of a convergent-divergent nozzle 26, in particular a de-Laval type nozzle. [0080] In a preferred embodiment, (see Fig. 5b, the system can include a cover means 35 that is configured to substantially close off or substantially cover a space which includes a treatment area A. Such a cover 35 can be made of non-transparent material, and allows for by- stander safety (avoiding stray laser light reading any by- standers). Besides, such a cover 35 can enhance fluid flow effects,concentrating ejected fluidF onto ortowards the treatment area A. [0082] Figures 3a,3b,3c show a furtheradvantageous example of the system 2, wherein it includes two arrays of optical elements 28, 28’, 30, 30’ for directing and fo- cusingthe respectivelaserbeams 8,8’inthe sameman- neras described above.In thisway,two respective treat- ment areas A, A’ can be treated, as is shown in Fig. 3c. Further, Fig. 3a, 3c show application of an air inlet 46 located upstream of a fluid flow generator 12 (e.g. fan or airpump),forintake ofambientairthatisto serve asfluid F to be ejected again via the first inner chamber 14 and laser beam slits 6,6’.As followsfrom fig.3c,in thiscase, each beampassage6,6’providesarespectivefluid con- nection 18, 18’ from the first space 14 into the external space19.Also,as followsfromFigure 3a,preferably,the two line-focused beams provide located substantially non-overlapping laser beam treatment areas A, A’ in the second focalplane.Moreover,again,a directionofmove- ment M of the system 2 with respect to the object 20 to be treated is preferablyperpendicularwith respect to the line-focused laser beam treatment areas A, A’, and is preferably in parallel with respect to center lines of the (in this case two) respective laser beam slits 6, 6’ [0083] The system2 can be configured orimplemented fortreating,in particular cleaning,a surface.Thesurface can e.g. be a running surface 34, of a rail 36 of a railway track 38 of a transport system 44, see figures 5a, 5b. In that case, the system 2 is preferably configured to be mounted on a train 40 (e.g. at a lower side or part of the train, facing a rail to be treated) which can run on the track 38, so that the system can treat the rail surface while the train 40 is, and preferably runs/moves, on the track 38. As is shown, the system can be configured to at least partly, preferably substantially, enclose a head section 42 of the rail 36 during operation, via an afore- mentioned cover or cover means 35. Thus, an embodi- ment also can provide a train 40 for treating, in particular cleaning, a rail 36, preferably two rails, of a railway track 38 on which the train 40 can run, in which case the train 40 is provided with one or more beam delivery systems 2 as described above.. [0084] Thus, use of the system 2 or a plurality of such systems 2 can include a method of treating, in particular cleaning, at least one rail 36, preferably two rails, of a railway track 38, in which case the at least one laser beam 8 can be directed to each rail 36 to be cleansed. In a preferred embodiment,themethod furthercomprises removing liquid, e.g. rain water, from the at least one rail (36), in particular from the running surface 34 of said rail 36, by a flow of gas, e.g. air. [0085] In this way, efficient and reliable surface treat- mentcanbeachieved,the systemhaving a long durability and low downtimes.In particular,the systemcan operate undervarious weatherconditions,andcan cope verywell wit with both dry and wet surfaces. [0086] The embodiments as disclosed are shown by way of example only and should not be construed as limiting the scope of the disclosure. Many variations are possible within the scope of the invention as defined by the claims. [0087] For example, the at least one laser beam may be focused on the at least one laser beam passage in multiple,e.g.two,spatialdimensions,wherein the optics assembly may be configured to e.g. produce a substan- tially point or circular type of focus of the at least one laser beam at the at least one beam passage. [0088] Thus, one of the first and second optical units 28,30 can be configuredtoproduce a line focus,wherein the other of the first and second optical units 28, 30 can [0081] As follows from Figures 1-2, during operation, the system can carry out a method wherein the la ser device 12 generates a laser beam 8. At the same time, a fluid flow F is generated by the respective fluid flow generating means 12.The laser beams 8 and fluid F exit the system via the elongated laser beam aperture (slit) 6, towards the surface to be treated. The laser beam 8 is being line-focused onto the elongated aperture 6 so that it substantially (preferably entirely) passes that ap- erture.Next,the laserbeamline-focuses onto the surface (at the respective second focal plane P2), to carry out a surface treatment. Debris that scatters from the treated surface has no or little chance of reaching the internal optics 28, 30 of the system due to the application of the shield 4 having the (relatively small, narrow) laser beam slit in combination with application of the fluid ejection. 7