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:
| Class | Firebox | Cylinder size | Valves | Valve gear | Number built | Years built |
| --- | --- | --- | --- | --- | --- | --- |
| E2 | Radial-stay | 20 ½” x 26” | Slide | Stephenson | 88 | 1901-02 |
| E2a | Belpaire | 20 ½” x 26” | Slide | Stephenson | 93 | 1902-05 |
| E2b | Belpaire | 20 ½” x 26” | Piston | Stephenson | 70 | 1903-04 |
| E2c | Belpaire | 20 ½” x 26” | Slide | Stephenson | 22 | 1903 |
| E2d | Belpaire | 20 ½” x 26” | Piston | Walschaerts | 32 | 1906-08 |
| E3 | Radial-stay | 22” x 26” | Slide | Stephenson | 8 | 1901-02 |
| E3a | Belpaire | 22” x 26” | Slide | Stephenson | 114 | 1903-05 |
| E3d | Belpaire | 22” x 26” | Piston | Walschaerts | 56 | 1906-10 |
| E7s | Belpaire | 22 ½” x 26” | Piston | Stephenson | 90 conv. fr. E2a,b,c | 1916-20 |
Question:
What was the engine class that was built on 1906-08?
Answer:
E2d