without shared rails:
+++++++++++++++++++++++++++++++++
| | | | |
| W | W | W | W |
| | | | |
---------------------------------
+++++++++++++++++++++++++++++++++
| | | | |
| W | W | W | W |
| | | | |
---------------------------------
+++++++++++++++++++++++++++++++++
| | | | |
| W | W | W | W |
| | | | |
---------------------------------
with shared rails:
+++++++++++++++++++++++++++++++++
| | | | |
| W | W | W | W |
| | | | |
---------------------------------
| | | | |
| M | M | M | M |
| | | | |
+++++++++++++++++++++++++++++++++
| | | | |
| W | W | W | W |
| | | | |
---------------------------------