Trem vs. comboio
The Portuguese words trem and comboio both translate to train in English, but their usage is one of the most well-known differences between Brazilian Portuguese and European Portuguese.
Trem
A1In Brazilian Portuguese, trem is the standard word for a train, including passenger trains, freight trains, and urban rail systems. In the state of Minas Gerais, Brazil, trem is also a very common slang word that can mean thing or stuff.
Eu vou pegar o trem para o Rio de Janeiro.
(I'm going to take the train to Rio de Janeiro.)
O trem de carga estava cheio de minério de ferro.
(The freight train was full of iron ore.)
As crianças acenaram quando o trem passou.
(The children waved when the train went by.)
Perdi o último trem da noite e tive que pegar um táxi.
(I missed the last train of the night and had to take a taxi.)
Pega aquele trem ali pra mim, por favor.
(Get that thing over there for me, please.)
Comboio
A1In European Portuguese (spoken in Portugal), comboio is the standard word for a train. In Brazilian Portuguese, however, comboio almost always means a convoy, which is a group of vehicles or ships traveling together, often for protection or mutual support.
Vou apanhar o comboio para o Porto amanhã de manhã.
(I'm going to catch the train to Porto tomorrow morning.)
A estação de comboios fica no centro da cidade.
(The train station is in the city center.)
O comboio de alta velocidade em Portugal chama-se Alfa Pendular.
(The high-speed train in Portugal is called Alfa Pendular.)
Um comboio de caminhões transportava a ajuda humanitária.
(A convoy of trucks was transporting the humanitarian aid.)
A polícia formou um comboio para escoltar o presidente.
(The police formed a convoy to escort the president.)
Summary
The main difference is geographic. Use trem for train when speaking Brazilian Portuguese and comboio for train when speaking European Portuguese. Be aware that in Brazil, comboio means convoy, and in some parts of Brazil, trem can also be slang for thing.