In this letter, the dependence of the specular (SMC) and dense multipath components (DMC) to the inter-user spatial correlation and sum-rate capacity of a massive multi-user MIMO (MU-MIMO) setup is evaluated from measurements conducted inside an underground subway wagon at the 25 -40 GHz candidate frequency bands for 5G systems.The radio channel consists in a 7 × 7 uniform rectangular array (URA) acting as access point (AP) for 8 users uniformly distributed in the wagon.The DMC power ratio is observed to be distance-and frequency-dependent as the SMC and DMC exhibit different propagation mechanisms.Remarkably, it is reported that the inter-user spatial correlation computed with DMC offers the best favorable propagation for a massive MIMO setup whereas SMC contribute to the users correlation.Hence, correlation is found to be strongly dependent on the DMC ratio frequency characteristics.In addition, better inter-user correlation and sumrate capacity values are obtained as the frequency is increased.These results highlight the need to include DMC in 5G massive MIMO channel models and emulators to improve their accuracy at the system level.