diff --git a/examples/nnx_toy_examples/03_train_state.py b/examples/nnx_toy_examples/03_train_state.py
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+++ b/examples/nnx_toy_examples/03_train_state.py
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+# Copyright 2024 The Flax Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# %%
+import jax
+import jax.numpy as jnp
+import matplotlib.pyplot as plt
+import numpy as np
+import optax
+
+from flax import nnx
+from flax.training import train_state
+
+X = np.linspace(0, 1, 100)[:, None]
+Y = 0.8 * X**2 + 0.1 + np.random.normal(0, 0.1, size=X.shape)
+
+
+def dataset(batch_size):
+  while True:
+    idx = np.random.choice(len(X), size=batch_size)
+    yield X[idx], Y[idx]
+
+
+class Linear(nnx.Module):
+  def __init__(self, din: int, dout: int, *, rngs: nnx.Rngs):
+    self.w = nnx.Param(jax.random.uniform(rngs.params(), (din, dout)))
+    self.b = nnx.Param(jnp.zeros((dout,)))
+
+  def __call__(self, x):
+    return x @ self.w.value + self.b.value
+
+
+class Count(nnx.Variable[nnx.A]):
+  pass
+
+
+class MLP(nnx.Module):
+  def __init__(self, din, dhidden, dout, *, rngs: nnx.Rngs):
+    self.count = Count(jnp.array(0))
+    self.linear1 = Linear(din, dhidden, rngs=rngs)
+    self.linear2 = Linear(dhidden, dout, rngs=rngs)
+
+  def __call__(self, x):
+    self.count.value += 1
+    x = self.linear1(x)
+    x = jax.nn.relu(x)
+    x = self.linear2(x)
+    return x
+
+class TrainState(train_state.TrainState):
+  counts: nnx.State
+  graphdef: nnx.GraphDef
+
+model = MLP(din=1, dhidden=32, dout=1, rngs=nnx.Rngs(0))
+graphdef, params, counts = nnx.split(model, nnx.Param, Count)
+
+state = TrainState.create(
+  apply_fn=None,
+  graphdef=graphdef,
+  params=params,
+  tx=optax.sgd(0.1),
+  counts=counts,
+)
+del params, counts
+
+
+@jax.jit
+def train_step(state: TrainState, batch):
+  x, y = batch
+
+  def loss_fn(params):
+    model = nnx.merge(state.graphdef, params, state.counts)
+    y_pred = model(x)
+    loss = jnp.mean((y - y_pred) ** 2)
+    counts = nnx.state(model, Count)
+    return loss, counts
+
+  grads, counts = jax.grad(loss_fn, has_aux=True)(state.params)
+  # sdg update
+  state = state.apply_gradients(grads=grads, counts=counts)
+
+  return state
+
+
+@jax.jit
+def test_step(state: nnx.TrainState[MLP], batch):
+  x, y = batch
+  model = nnx.merge(state.graphdef, state.params, state.counts)
+  y_pred = model(x)
+  loss = jnp.mean((y - y_pred) ** 2)
+  return {'loss': loss}
+
+
+total_steps = 10_000
+for step, batch in enumerate(dataset(32)):
+  state = train_step(state, batch)
+
+  if step % 1000 == 0:
+    logs = test_step(state, (X, Y))
+    print(f"step: {step}, loss: {logs['loss']}")
+
+  if step >= total_steps - 1:
+    break
+
+model = nnx.merge(state.graphdef, state.params, state.counts)
+print('times called:', model.count.value)
+
+y_pred = model(X)
+
+plt.scatter(X, Y, color='blue')
+plt.plot(X, y_pred, color='black')
+plt.show()