In many countries, railway is considered as the second source of ambient noise pollution, just after the noise from urban traffic. It is out of the question that the train noise is very annoying for the inhabitants in surrounding areas. The present study assessed noise pollution caused by passing inter-urban trains in southern Tehran using sound statistical parameters. In order to assess train noise pollution of areas surrounding the inter-urban railroads including residential and industrial places, as well as healthcare centers, a total number of 99 measurement stations were selected. To that end, the instruction present by EPA was followed up to measure maximum, minimum and equivalent sound level using sound level meter of TES 1358 type. Finally, a train noise index (TRNI) was derived from the relevant equations and field measurements. The obtained results revealed that the equivalent sound level was equal to 77.21 dB throughout the axis Tehran–Khorasan and 74.38 dB within the axis Tehran–Azerbaijan at all intervals of 15, 25, 50, 100 and 200 m from the railway. The investigations also suggested that with increasing distance from 15 to 200 m, the daily equivalent sound level decreases around 12.55 and 17.73 dB in axes Tehran–Khorasan and Tehran–Azerbaijan, respectively. The TRNI was also decreased equal to 18 and 16.53 dB in the axes Tehran–Khorasan and Tehran–Azerbaijan, severally. Involving multiple criteria including maximum, minimum and equivalent sound level, the TRNI is much more accurate than mere equivalent sound level so it would be more appropriate tool for noise pollution assessment.