be on a gravy train 走运，有赚钱的机会，gravy在美国口语中有"容易赚得的利润"的意思，gravy train意即"轻易发大财的工作".该习语直译为"在一辆容易赚钱的火车上",所以引申为"走运，有赚钱的机会".
a: john seems to be on the gravy train these days. 约翰最近看起来很走运。
b: yeah. he got a chance of going abroad to arrange an exhibition. 是啊。他得到了一个出国去办展览的机会。
a: i don't know why the sun is not shining on me. 我不明白为什么我的运气总是不佳。
b: don't worry. you'll be in the chips sooner or later. 别着急，总有一天你也会很富裕的。