The Helsinki commuter rail system comprises four local and four regional services, each of which are identified by letters of the alphabet.

Letters have been used as service identifiers since 1972. This was originally inspired by the Copenhagen S-train system. Unlike Copenhagen, which maintains a strict alphabetical order, letters of Helsinki commuter rail services are often assigned on the basis of mnemonics, such as E for Espoo and K for Kerava, with care taken to ensure that the original letters did not sound or look similar, to avoid confusion. In recent years, however, this rule has been broken, with the Y train to Siuntio and the R train now also serving Tampere.

Commuter train services operate on four railway lines, radiating from the Helsinki Central Station:

• westbound on the Coastal Line (A, E, H, L, U and Y)
• northbound on the Ring Rail Line (I/P, separating from the Main Line after Hiekkaharju station)
• northbound on the Main Line (K)

The VR commuter rail system consists of three local and four interurban services:

• The D and R and T run from Helsinki to Riihimäki, Hämeenlinna (D and R) and Tampere (R), with various stopping patterns.
• The Z runs northbound from Helsinki on the Main Line, but separates on to the Lahti Line to run northeast just after Kerava.
• The M forms Tampere's commuter rail. It runs from Nokia to Tampere, with some services extending to Toijala. As of August 2022, this now runs hourly throughout the day, with roughly two-hourly extensions to Toijala .
• The G runs hourly from Lahti to Riihimäki.
• The O runs mainly from Kotka Port to Kouvola, with some services running between Lahti and Kouvola/Kotka.

Frequencies are for off-peak only (Monday-Saturday).

Special lines are lines that do not run regularly, and are usually not listed (explicitly or even at all) on the HSL/VR map. These include the lines to Siuntio, Kouvola and Tampere.

These lines have both termini outside of the HSL area and include all lines from the VR commuter rail system.

• The M train runs from Nokia to Tampere and Toijala. It is Tampere's sole commuter rail line.
• The G train runs from Riihimäki to Lahti.
• The O train runs from Lahti to Kouvola/Kotkan satama and Kouvola to Kotkan satama.