Whether or not you can travel on a train without a ticket depends on the railway and the circumstances.

• Generally Not Allowed: In most countries and on most train lines, traveling without a valid ticket is not permitted. You are usually required to purchase a ticket before boarding or have a valid pass. Amtrak Rail Passes
• Exceptions: Some exceptions might exist. For example, you may be allowed to purchase a ticket on board if the ticket office at your origin station was closed, or if the train line allows it.
• Consequences: Traveling without a valid ticket usually carries consequences. You may be fined, asked to disembark at the next station, or in some cases, face legal action.

It is important to consult the specific rules and regulations of the train line you intend to use.