There are two ways to do it, without algebra, and with algebrs:

Method 1; Without algebra:

You can do it this way in you head.  When the faster train starts an 
hour after the slower train left, the slower train has 35 km head start.
The faster train's catch-up rate  is the difference in their speeds, or 
40-35 or 5 km/hr. So to catch up the entire 35 km at 5 km/hr will take 
7 hrs.   


Method 2: With algebra.

Make this chart:

                  Distance    Rate     Time
-------------------------------------------
Slower train     
Faster train       

Put in the speeds:

                  Distance    Rate     Time
-------------------------------------------
Slower train                   35
Faster train                   40         

Let t = the time it takes the faster train to catch
the slower train:

                  Distance    Rate     Time
-------------------------------------------
Slower train                   35       
Faster train                   40        t

Then since the slower train travels 1 hour more than
the faster train, we put t+1 for the time of the slower 
train:

                  Distance    Rate     Time
-------------------------------------------
Slower train                   35       t+1
Faster train                   40        t

Use Distance = Rate × time to indicate the multiplication
of the rate times the time.

                  Distance    Rate     Time
-------------------------------------------
Slower train      35(t+1)      35       t+1
Faster train        40t        40        t


Both trains had traveled the same distances when
the faster one catches the slower one.  So we
set the two distances equal:

 35(t+1) = 40t
35t + 35 = 40t
      35 = 5t
       7 = t

Answer: 7 hours.

Edwin