Meaning:

“满嘴” means “all his mouth,” and “跑火车” means “trains running around.” This slang term in Chinese is used derogatively to describe someone who likes to brag, exaggerate, lie or talk irresponsibly. The image of having trains running around in one’s mouth is vivid and reminds people of a similar saying in English that describes such a person as a “fast talker.”

Example:

A: 小马说他下个月打算去贵州山区小学当老师，我打算捐一些书让他带去。

Xiǎomǎ shuō tā xiàgeyuè dǎsuàn qù guìzhōu shānqū xiǎoxué dāng lǎoshī，wǒ dǎsuàn juān yīxiē shū ràng tā dàiqù。

Xiaoma said he plans to go to Guizhou next month and teach in a mountainous school there. I’d like to have him bring some books to the kids there.

B: 这家伙整天满嘴跑火车，你别太当真。

Zhè jiāhuo zhěngtiān mǎnzuǐ pǎohuǒchē ，nǐ bié tài dāngzhēn。

That guy is a fast talker. Don’t take him seriously.