Use \thickhline to draw a thick horizontal line.

\newcommand{\thickhline}{\noalign{\hrule height 0.8pt}}

Note that \hline twice does not suit your purpose.

An example of drawing a cute table.

\begin{table}
\centering
\begin{tabular}{l l l l l } \hline
SegID   & ProteinID & Loc   & Type  & Len \ \hline
1       & $S_1$     & 0     & $E$   & 3 \
2       & $S_1$     & 3     & $H$   & 2 \
3       & $S_1$     & 5     & $L$   & 2 \
4       & $S_1$     & 7     & $E$   & 3 \
5       & $S_2$     & 0     & $L$   & 2 \
6       & $S_2$     & 2     & $E$   & 3 \
7       & $S_2$     & 5     & $H$   & 1 \ \hline
\end{tabular}\
\caption{Segment table for two sequence, $S_1$ = `$EEEHHLLEEE$' and
    $S_2$ = `$LLEEEH$'}
\end{table}