你可以这样说：
"She was reading a book on the subway."
"She was reading a book while on the subway."
"She was reading a book during her ride on the subway."
这些短语都可以用来描述她在地铁上看书的情况。
关于这些短语的来源：
"On the subway" 是指在地铁上，它是一个常用的短语，用来描述一个人在地铁上的位置或情况。
"While on the subway" 和 "during her ride on the subway" 都是表示时间的短语，用来描述一个人在地铁上的时间段。"While" 表示一段时间的过程，"during" 表示一段时间的结果。
下面是一些其他的英文例句，包含了这些短语的更多用法：
She was reading a book on the subway to pass the time. (她在地铁上看书打发时间。)
I saw her reading a book on the subway when I got on. (我在上车的时候看到她在地铁上看书。)
She reads a lot on the subway because she has a long commute. (她在地铁上看很多书，因为她的通勤很长。)
He was reading a book while on the subway, but he fell asleep halfway through. (他在地铁上看书，但是他半途睡着了。)
She always brings a book to read during her ride on the subway. (她总是带一本书在地铁上看。)
"She was reading a book on the subway."
"She was reading a book while on the subway."
"She was reading a book during her ride on the subway."
这些短语都可以用来描述她在地铁上看书的情况。
关于这些短语的来源：
"On the subway" 是指在地铁上，它是一个常用的短语，用来描述一个人在地铁上的位置或情况。
"While on the subway" 和 "during her ride on the subway" 都是表示时间的短语，用来描述一个人在地铁上的时间段。"While" 表示一段时间的过程，"during" 表示一段时间的结果。
下面是一些其他的英文例句，包含了这些短语的更多用法：
She was reading a book on the subway to pass the time. (她在地铁上看书打发时间。)
I saw her reading a book on the subway when I got on. (我在上车的时候看到她在地铁上看书。)
She reads a lot on the subway because she has a long commute. (她在地铁上看很多书，因为她的通勤很长。)
He was reading a book while on the subway, but he fell asleep halfway through. (他在地铁上看书，但是他半途睡着了。)
She always brings a book to read during her ride on the subway. (她总是带一本书在地铁上看。)