I have a question to merge two columns into one in the same dataframe(start\_end), also remove null value. I intend to merge 'Start station' and 'End station' into 'station', and keep 'duration' according to the new column 'station'. I have tried pd.merge, pd.concat, pd.append, but I cannot work it out. 

dataFrame of Start\_end:

```
    Duration    End station     Start station
14  1407        NaN             14th & V St NW
19  509         NaN             21st & I St NW
20  638         15th & P St NW.  NaN
27  1532        NaN              Massachusetts Ave & Dupont Circle NW
28  759         NaN              Adams Mill & Columbia Rd NW

```

Expected output: 

```
    Duration    stations
14  1407        14th & V St NW
19  509         21st & I St NW
20  638         15th & P St NW
27  1532        Massachusetts Ave & Dupont Circle NW
28  759         Adams Mill & Columbia Rd NW

```

Code i have so far:

```
#start_end is the dataframe, 'start station', 'end station', 'duration'
start_end = pd.concat([df_start, df_end])

```

This is what I attempted to:

```
station = pd.merge([start_end['Start station'],start_end['End station']])

```