PLC is a concept born at the beginning of the 20th century, developed for signaling and control operations [26]. As mentioned above, it refers to technology that allows for data communication over existing electrical power lines, providing for the simultaneous transfer of electrical power and data. PLC technologies effectively utilize the existing infrastructure, enabling a cost-efficient and streamlined deployment approach. They represent an appealing alternative to those solutions that necessitate the installation of new networks for several reasons. First of all, the use of an already installed cable network allows cost and installation time reductions in the deployment of the communication system [27]. PLC solutions are not affected by the typical problems of systems that exploit the free propagation of the electromagnetic signal, which means they perform better in those scenarios of the rural area or where obstacles limit wireless communications [28]. In fact, wireless setups are generally more delicate and vulnerable than power lines [29]. It is worth highlighting that the aforementioned issues have already paved the way to the implementation of communication channels that are integrated into the existing electrical transmission and distribution network. These communication channels have shown to be a viable solution for the service providers to afford important managing tasks and meet the stringent service delivery demands. In a believable way, for a wide range of application scenarios, including railways, the PLC technology still has a role to play.

PLC technologies can be classified based on the characteristics of the underlying electrical system and/or specific features as described in the following.

Current regime: It is a distinction based on the utilization of the direct current (DC) or alternating current (AC) regime. The first is often preferred due to its simpler structure as the AC regime requires phase and reactive power control; for PLC applications, the installation seems facilitated upon lines operated in the DC regime [30].

Nominal voltage: It is a distinction based on the operating voltage value of the power grid on which the PLC is installed. According to the European norm EN 50160 [31], power grids can be classified as low-voltage (LV) if the nominal voltage is less than 1 kV, medium voltage (MV) if it is within the range (1–35) kV, and high voltage (HV) if it is above 35 kV. LV and MV power grids are generally used for electricity supply in urban and suburban areas, while HV grids are used for long-distance power transmission.

Frequency range: PLC technologies can be categorized according to the range of operating frequency values of transmitted signals [32]. This feature is indicative of data transmission speed and distance. According to the literature, the frequency range in which PLC systems can operate covers frequencies from 125 Hz to 100 MHz [33,34,35]. PLC systems are usually referred to as shown in Table 1:

Ultra-narrowband	Identified by the operating frequency range (125–3000) Hz. Ultra-narrowband PLC (UN-PLC) technology typically guarantees transmission over long distances, although the data rate is typically in the order of kbps [28]. Examples of UN-PLC are the home automation system by Pico Electronics X10 deployed since 1975 [34] and the two-way automatic communication system (TWACS) by Aclara [28].
Narrowband	Identified by the operating frequency range (3–500) kHz. Narrowband PLC (NPLC) technologies are characterized by variable data rates from 1 kbps up to 1 Mbps and are capable of guaranteeing medium/long transmission distances [28]. In Europe, they are distinguished into four frequency bands, standardized by the European Committee for Electrotechnical Standardization (CENELEC) in 1992 [28]: CENELEC-A (3–95 kHz), CENELEC-B (95–125 kHz), CENELEC-C (125–140 kHz), and CENELEC-D (140.0–148.5 kHz). According to the data transmission rate, NPLC systems are also referred to as low data rate (LDR) and high data rate (HDR). LDR systems adopt single-carrier modulations, whereas HDR exploit multi-carrier modulations to achieve data rates up to 1 Mbps [28].
Broadband	Characterized by a data rate above 200 Mbps and shorter transmission distance than NB-PLC [14]. The operating frequency range is not uniformly established: according to the Institute of Electrical and Electronic Engineers (IEEE) [35], it ranges from 1.8 MHz up to 100.0 MHz. The International Telecommunication Union (ITU) instead specifies a frequency range up to 300 MHz [36]; the standards developed by the HomePlug Powerline Alliance use the frequency ranges (1.8–30.0) MHz and (1.8–86.0) MHz [36].

3. Further Remarks