During laser-powder bed fusion (L-PBF) the printed material is subjected to multiple fast heating and cooling cycles when the laser interacts with neighboring tracks or layers above. The complex thermal history influences the final microstructure and the macroscopic properties of the printed part. In this work, we demonstrate how high-speed in situ X-ray diffraction in transmission mode can be used to measure temperature profiles and cooling rates in a Ti-6Al-4V single-track wall. During the laser remelting of the top layer, a temperature exceeding the β transus temperature (T_β ∼ 1252 K) is measured up to 150 µm below the surface. The maximum observed cooling rates vary from 10⁶ K/s at the top surface, to 10⁵ K/s at a depth of 135 µm and 10⁴ K/s at a depth of 255 µm. Based on the temporal evolution of the various crystallographic phases, the dimensions of the melt pool and the high-temperature β zone surrounding the melt pool are estimated. It is anticipated that the data obtained from in situ measurements in transmission mode on a thin wall combined with in situ measurements in reflection mode on a bulk sample will allow verification and validation of finite element models used in L-PBF processing.