電車の中は暖かいって英語でなんて言うの？

This phrase is providing a difference in the temperatures by noticing that outside is colder than inside. By adding 'luckily' one is stating that they are happy to be the one that is inside the train. Various words can be used instead of cold. The first phrase you do not need to state that you are on the train as it is implied within the sentence.

"It is freezing outside but hot in the train!" "freezing" meaning that it is very cold outside "It is very hot in the train, I have to remove my outerwear!" Normally when it is cold outside, the heating system in the train is set to a warmer temperature, which means that I have to remove my jacket and muffler.

The example above would be a good way of expressing to someone that the train is nice and warm while its cold outside.

When saying the train is warm inside you may simply say something like "It is nice and warm inside the train." or "The train is nice and warm inside.". Both sentences describe the temperature inside the train and should you compare it to the outside you may say something like "It is nice and warm in the train unlike the cold outside."

We have so many quaint expressions in English... Idioms, and similes that paint a picture... This is called figurative language... and the British love it! When its cold outside one might remark on the contrast;-) EG: it's cold out there and so toasty inside the train...I am as warm as toast even though its freezing outside!

If you're cold, the train will warm you up. / The train will warm you up if you're cold. - These sentences can be used to express that the train is warm and it will increase the temperature of people who enter it if they're cold. In these sentences, the phrase "warm you up" means "make you warm". For example, If you're cold sit closer to the campfire, it'll "warm you up" = If you're cold sit closer to the campfire, it'll "make you warm" "Warm up" by the heater if you're cold. = "Make yourself warm" by the heater if you're cold. The phrase "warm you up" can also mean "make you happy" if you're feeling negative. For example, My friends always "warm me up" when I'm feeling sad. = My friends always "make me happy" when I am feeling sad.

When travelling on a bus or train wth alot of people then it can feel really warm with everyone's body heat you can explain this by saying 'it's warm inside the train' or 'inside the train is warm'

It's cold outside, but it's warm inside the train. You can explain that it's warm inside the train by using one of the above suggestions.To add emotion and meaning to your sentence you may choose to give an opinion or note of approval about this contrast in temperatures - as in the first example. The speaker in the second example seems to disapprove of both these extremes.

To compare the warmth between the train and the outdoors, you describe one as being "warmer" or "colder" than the other. So, you can say "it's (it is) warmer inside the train than it is outside." Usually when comparing two things, you use the simple construction of "(noun) is (adjective +er) than (noun). Or you can use "(noun) is more (adjective) than (noun)." For example: It's warmer inside the train than outside. It's more warm inside than outside. It's colder outside than on the train. It's more cold outside than it is on the train.