Some websites, there are quite a few actually that do this for you so for example Rail Easy is a really good website that'll do it for you and calculate whether or not it'd be cheaper; and don't worry this is completely within the terms of use from these rail companies, you're not breaking any rules.
其實很多網站都有分段購票的服務,例如 Rail Easy 是一個非常好的網站,它可以計算票價是否變得更便宜,且不用擔心違反鐵路公司的任何規定。