Rail measuring system comprising: a rail with a longitudinal extension and a neutral axis which extends along the longitudinal extension, on which rail a mechanical force acts which is generated by a train running thereon; and at least one fiber-optic sensor unit (303, 330, 330a, 330b) for detecting a mechanical force that acts on the rail, comprising: an optical fiber; a fiber Bragg grating provided in the optical fiber which has a Bragg wavelength that depends on the mechanical force; a converter structure, wherein the converter structure has a signal amplification lever; and an edge filter for filtering a first part of the signal light, in particular wherein the edge filter has a filter characteristic line with a nominal slope of 8 % per nm related to the transmitted intensity or less, in particular between 2 % and 7 % per nm, in particular wherein a measuring range of axle loads of 200 kg to 50,000 kg that act on the rail is provided, wherein the converter structure is designed to provide temperature compensation, wherein the thermal expansion coefficient of the converter structure of a converter structure with negative transmission ratio is smaller than the thermal expansion coefficient of the rail or wherein the thermal expansion coefficient of the converter structure of a converter structure with positive transmission ratio is larger than the thermal expansion coefficient of the rail, and wherein the at least one fiber-optic sensor unit is mounted to the neutral axis. a rail with a longitudinal extension and a neutral axis which extends along the longitudinal extension, on which rail a mechanical force acts which is generated by a train running thereon; and at least one fiber-optic sensor unit (303, 330, 330a, 330b) for detecting a mechanical force that acts on the rail, comprising: an optical fiber; a fiber Bragg grating provided in the optical fiber which has a Bragg wavelength that depends on the mechanical force; a converter structure, wherein the converter structure has a signal amplification lever; and an edge filter for filtering a first part of the signal light, in particular wherein the edge filter has a filter characteristic line with a nominal slope of 8 % per nm related to the transmitted intensity or less, in particular between 2 % and 7 % per nm, in particular wherein a measuring range of axle loads of 200 kg to 50,000 kg that act on the rail is provided, wherein the converter structure is designed to provide temperature compensation, wherein the thermal expansion coefficient of the converter structure of a converter structure with negative transmission ratio is smaller than the thermal expansion coefficient of the rail or wherein the thermal expansion coefficient of the converter structure of a converter structure with positive transmission ratio is larger than the thermal expansion coefficient of the rail, and wherein the at least one fiber-optic sensor unit is mounted to the neutral axis. an optical fiber; a fiber Bragg grating provided in the optical fiber which has a Bragg wavelength that depends on the mechanical force; a converter structure, wherein the converter structure has a signal amplification lever; and an edge filter for filtering a first part of the signal light, in particular wherein the edge filter has a filter characteristic line with a nominal slope of 8 % per nm related to the transmitted intensity or less, in particular between 2 % and 7 % per nm, in particular wherein a measuring range of axle loads of 200 kg to 50,000 kg that act on the rail is provided, wherein the converter structure is designed to provide temperature compensation, wherein the thermal expansion coefficient of the converter structure of a converter structure with negative transmission ratio is smaller than the thermal expansion coefficient of the rail or wherein the thermal expansion coefficient of the converter structure of a converter structure with positive transmission ratio is larger than the thermal expansion coefficient of the rail, and wherein the at least one fiber-optic sensor unit is mounted to the neutral axis.