On two parallel lines, the distinct points $A_1,A_2,A_3,\ldots $ respectively $B_1,B_2,B_3,\ldots $ are marked in such a way that $|A_iA_{i+1}|=1$ and $|B_iB_{i+1}|=2$ for $i=1,2,\ldots $. Provided that $A_1A_2B_1=\alpha$, find the infinite sum $\angle A_1B_1A_2+\angle A_2B_2A_3+\angle A_3B_3A_4+\ldots $
1. **Define the points and distances:**
   - Points \( A_1, A_2, A_3, \ldots \) are on one line with \( |A_iA_{i+1}| = 1 \).
   - Points \( B_1, B_2, B_3, \ldots \) are on a parallel line with \( |B_iB_{i+1}| = 2 \).
   - Given \( \angle A_1A_2B_1 = \alpha \).

2. **Introduce auxiliary points:**
   - Let \( C_1, C_2, \ldots \) be points on the same line as the \( B \)'s such that \( C_1 = B_1 \) and \( |C_iC_{i+1}| = 1 \).

3. **Sum of angles:**
   - We need to find the infinite sum \( \angle A_1B_1A_2 + \angle A_2B_2A_3 + \angle A_3B_3A_4 + \ldots \).

4. **Transform the problem:**
   - Consider the angles \( \angle A_1C_1A_2 + \angle A_2C_2A_3 + \angle A_3C_3A_4 + \ldots \).
   - Let \( D_i \) be the intersection of \( A_2C_1 \) and \( A_1C_i \).

5. **Parallelogram property:**
   - Since \( A_1A_2C_{i+1}C_i \) forms a parallelogram, we have \( \angle A_1C_1A_2 + \ldots + \angle A_1C_iA_2 = \angle A_1D_iA_2 \).

6. **Limit of the angle sum:**
   - As \( i \to \infty \), the angle \( \angle A_1D_iA_2 \) approaches \( 180^\circ - \alpha \).

7. **Conclusion:**
   - Therefore, the infinite sum of the angles is \( 180^\circ - \alpha \).

The final answer is \( \boxed{180^\circ - \alpha} \).