For someone involved on “the other side”, the general process is as follows:

After a member of revenue protection identifies a suspected fare evader, a report is submitted to the prosecution department of the relevant TOC (Train Operating Company).

The TOC has up to 6 months from the date of the incident to initiate legal proceedings. Generally they will follow this process:

Firstly, they will send out an initial letter which states the alleged offence and asks for the suspected fare evader to provide their side of the story. Often, if a suspect shows a willingness to participate constructively in the process and show genuine remorse for their actions, the TOC will offer them an out of court settlement. This feels like a fine, but isn’t. It is a sum paid to the TOC (Normally the full unpaid fare + £100 - £150 of administrative costs).
If the suspect is not offered an out of court settlement, they will be summoned to court. The next stage will depend on which legislation they are prosecuted under.

For a Byelaw offence (the less serious of the two), a SJPN (single justice procedural notice) is sent to the suspect, asking for the suspect to plead either guilty or not guilty. If the defendant pleads guilty, they will receive a 33% reduction on the charges imposed by the court. A guilty plea also means that the defendant doesn’t need to attend court and the sentence is decided by the magistrates in their absence. The sentence is almost always a charge comprising of a fine, victim surcharge and admin costs. If they plead not guilty, they will be sent a summons and required to attend court and get a solicitor to represent them. The prosecution will then need to prove beyond reasonable doubt that the suspect was caught travelling without a valid ticket.

For the more serious offence (regulation of railways act, generally reserved for multiple-time offenders or those who have shown clear intent to avoid their fare) - generally no SJPN is issued and the defendant is required to attend a court hearing where they will enter a plea and face a (generally higher) charge.

A byelaw offence is immediately “spent” and won’t appear on a standard DBS check (so basically, it is immediately struck off of your criminal record).

For a ROR act prosecution, the offence remains on your criminal record for one year.