Questions about extract-lme.cov

I am using extract.lme.cov to extract the covariance matrix of lme. But
the results are not expected. 

 b <- lme(travel~1,Rail,~1|Rail)

The default correlation for lme is no correlation within groups.

extract.lme.cov(b,Rail)

The part of covariance matrix looks like:

         1        2        3        4        5        6
1 631.4778 615.3111 615.3111   0.0000   0.0000   0.0000
2 615.3111 631.4778 615.3111   0.0000   0.0000   0.0000
3 615.3111 615.3111 631.4778   0.0000   0.0000   0.0000
4   0.0000   0.0000   0.0000 631.4778 615.3111 615.3111
5   0.0000   0.0000   0.0000 615.3111 631.4778 615.3111
6   0.0000   0.0000   0.0000 615.3111 615.3111 631.4778

Where does the covariance come from? Maybe I'm missing sth here?