Is the word trens the same in Brazilian and European Portuguese?

No.

The difference lies in typical usage. While the spelling and the core meaning (rail vehicles) are the same, the word is not used interchangeably in daily life. In Brazil, trens (the plural of trem) is the standard, everyday term. In Portugal, the standard word is comboios. While a Portuguese person would understand "trens," they would identify it immediately as a Brazilianism and would almost exclusively use "comboios" when referring to the railway system. Additionally, the pronunciation of the final "s" differs: in most Brazilian regions, it is a soft "s" [s], whereas in Portugal, it is typically a "sh" sound [ʃ].

Brazilian Portuguese examples:

1. Os trens de São Paulo costumam ficar muito cheios. (The trains in São Paulo usually get very full.)
2. Os trens de carga atravessam o país inteiro. (The freight trains cross the entire country.)
3. Não gosto de andar nesses trens velhos. (I don't like riding in these old trains.)
4. Os trens de subúrbio estão com atrasos hoje. (The suburban trains are experiencing delays today.)
5. Os novos trens são muito mais confortáveis. (The new trains are much more comfortable.)

Continental Portuguese examples:

1. Os comboios de Lisboa costumam ficar muito cheios. (The trains in Lisbon usually get very full.)
2. Os comboios de mercadorias atravessam o país inteiro. (The freight trains cross the entire country.)
3. Não gosto de andar nestes comboios velhos. (I don't like riding in these old trains.)
4. Os comboios suburbanos estão com atrasos hoje. (The suburban trains are experiencing delays today.)
5. Os novos comboios são muito mais confortáveis. (The new trains are much more comfortable.)