15  EP 3 800 002 A1 16  - the means (12) for providing a fluid flow (F) through the at least one beam passage (6) are configured to provide a fluid pressure gradient between opposite sides of the shield (4), in par- ticulara fluidpressuregradientalong the atleast one beam passage (6); and/or - the means (12) for providing a fluid flow (F) through the at least one beam passage (6) are configured such that the provided fluid flow (F) issubstantially directed inamainbeamdirection (B) of the laser beam (8), preferably at a main flow velocityofatleast100 m/s,more preferably at least 200 m/s, for example about 250 m/s. or a set of lenses and/or mirrors, for focusing the laser beam (8) with respect to a first spatial direction (D1) which is substantially transverse to a second spatial direction (D2), wherein a mainfocallengthofthe firstopticalunitsubstan- tially corresponds to a distance (S1) between thefirstopticalunit(28)and the respectivebeam passage (6); and -a second opticalunit(30),e.g.a lens ora mirror or a set of lenses and/or mirrors, for focusing the laser beam (8) with respect to the second spatial direction (D2), wherein a main focal length of the second optical unit substantially correspondstoa distance(S2)betweenthesec- ond optical unit (30) and a treatment area (A), -the atleastone beampassage (6)iselongated, wherein a length (L) of the at least one beam passage (6) is atleast three times,preferably at least four times, more preferably at least five times, larger than a width (W) of said passage (6); and/or - the at least one beam passage (6) includes or provides a constriction (24) in a fluid flow path through the beam passage (6), wherein in par- ticular the beam passage (6) includes and/or is part of a convergent-divergent nozzle (26), in particular a de-Laval type nozzle; and/or - the or each beam passage (6), in particular a respective relatively narrow width (W) thereof, isdimensionedtoconform to arespectivebeam width of the or each respective laser beam (8), in particular a beam width in the range from the 1/e$^{2}$ beam width (BW1) to the 99.99% intensity beam width (BW2). - a first optical unit (28), e.g. a lens or a mirror wherein preferably at least one, more preferably each, of the first and second optical units (28, 30) is configured to produce a line focus and preferably includes a substantially cylindrical lens or mirror. 9. System according to any of the preceding claims, wherein the optics assembly (10)comprises,prefer- ably for each laser beam (8) of the at least one laser beam (8): 8. System according to any of the preceding claims, wherein: 7. System according to any of the preceding claims, wherein the optics assembly (10) is configured to at least partly, preferably substantially, conform the at least one laser beam (8) to the at least one beam passage (6). 6. System according to any of the preceding claims, wherein the system (2) further comprises at least one laser (22) for generating the at least one laser beam (8), to be transmitted through the beam pas- sage (6) by the optics assembly (10), wherein the at least one laser (22) preferably includes at least one pulsed laser,in particular having a laser wavelength of about 1064 nm. 10. System according to any of the preceding claims, wherein the systemincludes a covermeans (35)that isconfigured to substantiallyclose offorsubstantially cover a space which includes a treatment area (A). 11. System according to any of the preceding claims, including a plurality of, e.g. two, beam passages (6, 6’) which are associated with a respective plurality of laser beams (8, 8’). 12. System according to any of the preceding claims, wherein the system is configured fortreating, in par- ticularcleaning,a surface,in particulara running sur- face (34),ofarail(36)ofa railwaytrack (38),wherein thesystem(2)isconfigured to be mounted on a train (40) which can run on the track (38) and to treat the surface while the train (40)is,preferably runs,on the track (38), wherein preferably the system is configured to at leastpartly,preferably substantially,enclose a head section (42) of the rail (36) during operation. 13. Train (40) for treating, in particular cleaning, at least one rail (36), preferably two rails, of a railway track (38)onwhich the train (40)can run,whereinthe train (40) is provided with at least one beam delivery sys- tem(2)according to claim12,preferably atleastone respective system (2)foreach rail(36)to betreated. 14. Transport system for operating a train service, wherein the transport system (44) comprises a rail- way track (38) or a network of railway tracks and at least one train (40) according to claim 13 fortreating one or more rails (36) of the railway track (38) or the network of railway tracks. 15. Method oftreating,in particularcleaning,atleastone 9