Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| County | Tunnel | Type | Length (Imperial) | Date of opening |
| --- | --- | --- | --- | --- |
| Londonderry | Castlerock | Rail | 668 yd | 1845 - 1853 |
| Londonderry | Downhill | Rail | 307 yd | 1845 - 1846 |
| Armagh | Lissummon | Rail | 1759 yd | early 1860s |
| Down | Binnian Tunnel | Aqueduct | 2.5 miles | 1948 - 1952 |
| Antrim | Whitehead | Rail | 145 yd | 1862 - 1994 |
| Tyrone | Dungannon | Rail | 800 yd | 1862 |
Question:
What Tunnel has an Imperial Length of 307 yd?
Answer:
Downhill